HIV: GLOSSARY

ACTG: ACTG stands for AIDS clinical trial groups (also called ACTU, with “units” instead of “groups”). ACTGs are a consortium of medical centers throughout the United States that conduct clinical trials of drugs for treating people with HIV infection. Specifically, the drugs are for treating HIV itself, for treating opportunistic infections or tumors, and for stimulating the immune system. ACTGs are funded federally through the National Institutes of Health. Taken together, the ACTGs are the largest trials of drugs for treating HIV infection in the world: they have the largest budgets, the largest number of investigators, and the largest number of participants.
AIDS: AIDS stands for acquired immune deficiency syndrome. AIDS is the late stage of an infection caused by the human immunodeficiency virus, or HIV. The virus infects the CD4 cell (also called a T4 cell, a T4 lymphocyte, and a T-helper cell), which is critical to immune
defenses. As the numbers of these cells decrease, the immune system weakens until it becomes susceptible to what are called opportunistic infections and opportunistic tumors. These infections and tumors are called opportunistic because the microbes that cause them are opportunists, taking advantage of a weakened immune system. The person with AIDS is not susceptible to all infections: CD4 cells are responsible for only a certain part of the immune defenses. As a result, the person with AIDS does not get colds or flus any more frequently than anyone else; and the opportunistic infections people with AIDS do get either do not occur in healthy persons or cause illnesses that are far less severe. Opportunistic tumors are tumors that occur disproportionately frequently in persons with fewer CD4 cells.
AIDS-advocacy organizations: See Community-based organizations, below.
AIDS-defining diagnosis: A person with an AIDS-defining diagnosis has HIV infection plus an immune system weakened enough to allow one of several opportunistic infections or tumors to occur. As a result, the person is now said to have AIDS. The opportunistic infections and tumors that make up the AIDS-defining diagnoses, according to the Centers for Disease Control, include Pneumocystis pneumonia, Kaposi’s sarcoma, toxoplasmic encephalitis, cryptococcal meningitis, candidal esophagitis, infection with cytomegalovirus throughout the body, and Mycobacterium avium-intracellulare infections throughout the body. These are not the only AIDS-defining diagnoses; they are simply the most common.
AIDS dementia complex: AIDS dementia complex is the term used for the dementia that appears to result from HIV’s directly infecting the brain. Dementia means the loss of intellectual capacity, including the loss of memory, judgment, or concentration. AIDS dementia complex is relatively common in HIV infection, but usually only in the late stages.
AIDS-related complex: See ARC, below.
Amitriptyline: Amitriptyline hydrochloride (or, e.g., Elavil) is one of a group of drugs called tricyclic antidepressants that are grouped together because of their chemical similarities. Other tricyclic antidepressants include amoxapine (or Asendin), desipramine hydrochloride (or Pertofrane or Norpramin), doxepin hydrochloride (or Adapin or Sinequan), imipramine hydrochloride (or, e.g., Tofranil), and nortriptyline hydrochloride (or Aventyl Hydrochloride or Pamelor). Tricyclic antidepressants are used to treat depression and the peripheral neuropathy that causes painful feet. In many cases, the dose is arbitrary: many people start on a low dose and have the dose increased as necessary. Side effects are common, but usually not severe enough to stop treatment. The main side effects are drowsiness, weakness, and fatigue; dry mouth; constipation; and low blood pressure and dizziness. These side effects are all dose-related,
meaning the higher the dose, the more severe the side effect. Because the drugs cause drowsiness, they are often given before bedtime.
Amphotericin B: The antibiotic amphotericin B is the standard treatment for many infections caused by fungi, including most of the fungi that affect people with HIV infection: Candida, Cryptococcus, Histoplasma, Coccidioides, and Aspergillus. Amphotericin B, which is given only by vein, is highly effective. Unfortunately, it is also one of the most toxic antibiotics known. The most important side effects include kidney damage, anemia (see below), disturbances in the
balance of electrolytes, nausea and vomiting, fever and chills, and phlebitis or inflammation of the vein into which the drug is injected. Many of these side effects can be reduced in severity or eliminated by stopping the drug, by continuing the drug at a lower dose, or by giving other medications at the same time that will counteract the side effects. Because of amphotericin B’s toxicity, other drugs, like ketoconazole and fluconazole, are given when they are considered to be as effective or nearly as effective.
Anemia: Anemia means that the number of red cells in the blood is reduced. Red blood cells are responsible for delivering oxygen to all parts of the body. When the reduction is severe, the result is fatigue. Anemia can be caused by HIV infection itself, by an opportunistic infection, or by several of the drugs commonly taken by people with AIDS. Drugs often responsible include trimethoprim-sulfamethoxazole, other sulfa drugs, pentamidine, amphotericin B, and AZT. When the anemia is severe, it can be corrected with transfusions. When drugs are responsible, the drugs can be reduced in dose or discontinued.
Antibiotics: Antibiotics are drugs made from natural substances (as opposed to drugs made artificially) that inhibit the growth of microbes. Antibiotics may be effective against any of the classes of microbes—including bacteria, fungi, parasites, and viruses—that cause infections. Common examples of antibiotics frequently used in people with HIV infection include trimethoprim-sulfamethoxazole, other drugs containing sulfas, pentamidine, ketoconazole, amphotericin B, pyrimethamine, ganciclovir, acyclovir, penicillin, erythromycin, nystatin, clotrimazole, and AZT.
Antibody: Antibodies are proteins and are the part of the complex immune system that attacks any substance—protein or microbe—that is foreign to the body. Certain cells called B lymphocytes recognize these substances as foreign and manufacture antibodies that inactivate or eliminate the foreign substance. The foreign substance that the antibodies attack is called an antigen (see below). For most antigens, the B lymphocytes take one or two weeks to produce antibodies; for HIV, however, the time required may be months.
Antigen: Antigens are foreign material, including microbes, that the immune system responds to by manufacturing antibodies.
Aphthous ulcer: Aphthous ulcers are ulcers or sores in the mouth and occasionally in the esophagus. They are often extremely painful, they have no clear cause, and they are often cleared up by corticosteroids (see below) or other medications.
ARC: ARC stands for AIDS-related complex. ARC is a collection of conditions associated with HIV infection that do not meet the diagnostic definition of AIDS. There is no official definition of ARC. ARC is used to refer to symptoms that often, but not invariably, mean the immune system is weakening; therefore ARC often but not invariably means that the person will soon have AIDS. Common conditions of ARC include thrush, oral hairy leukoplakia, weight loss, chronic fever, chronic fatigue, and chronic diarrhea. These conditions should be accompanied by HIV infection and should have no other likely explanation. Because some of these conditions are vague and subjective, and because they may or may not mean the person will soon have AIDS, ARC is often viewed as a nebulous term.
Asymptomatic: Asymptomatic means the absence of symptoms. The asymptomatic person feels healthy.
B lymphocytes: B lymphocytes are the white blood cells—called lymphocytes—responsible for producing antibodies. B lymphocytes are distinct from T lymphocytes (including CD4 cells, also called T4 cells), which are also part of the immune system, but which work against a different group of microbes using different mechanisms.
Baclofen: Baclofen is a drug used to control muscle spasms. The most common side effect is drowsiness and, in large doses, severe sedation, lack of coordination, and lowered functioning of the heart and lungs.
Barbiturates: Barbiturates are drugs commonly used to treat insomnia, anxiety, and seizures. Examples of barbiturates are amobarbital (or Amytal), pentobarbital (or Nembutal), phenobarbital, and secobarbital. All barbiturates affect the central nervous system: low doses cause mild sedation, and high doses can lead to deep coma. When barbiturates are used for sedation, they remain effective for only about two weeks. As a result, alternative drugs are generally preferred to treat insomnia. Barbiturates’ most important role may be for controlling anxiety.
The major side effects are symptoms of central nervous system depression, including drowsiness, depression, lethargy, and hangovers. People who take barbiturates should be aware that the drug may impair their ability to perform hazardous activities. Prolonged use of high doses of the drug can cause physical dependence, psychological dependence, and tolerance (that is, higher doses of the drug are required to produce a similar effect). Discontinuing barbiturates can cause withdrawal symptoms that are similar to the withdrawal symptoms an alcoholic has when abruptly discontinuing alcohol. Other side effects include stomach pain, allergic reactions, and fever.
Benzodiazepines: Benzodiazepines are a class of drugs commonly used to treat anxiety, insomnia, seizures, and painful muscles. Examples of benzodiazepines include alprazolam (or Xanax), diazepam, flurazepam hydrochloride (or Dalmane), lorazepam (or Ativan), midazolam maleate, oxazepam (or Serax), prazepam (or Centrax), temazepam (or Restoril), and triazolam (or Halcion). In general, all benzodiazepines act in similar ways and seem to be equally effective. Most physicians prefer benzodiazepines for treating anxiety and tension. Compared to barbiturates and meprobamate, and when given at the doses that relieve anxiety, they are less addictive and produce less sedation. The major side effects are drowsiness, loss of coordination, confusion, dizziness, and fainting. People taking benzodiazepines should be aware that the drug may impair their ability to perform activities that require mental alertness and physical
coordination. Benzodiazepines can also cause physical dependence and symptoms of severe withdrawal if the drug is stopped suddenly after being used regularly for a long time.
Biopsy: Biopsy is a procedure for obtaining a piece of tissue for examination under the microscope. The microscopic changes in tissue often provide a diagnosis, and stains and cultures for microbes will often reveal the infecting organism. The biopsy may be obtained using lidocaine to deaden the skin to avoid pain. The biopsy may be performed on an outpatient basis when the area to be biopsied is near the surface or when it is in the lungs or gastrointestinal tract and can be reached through an endoscope, an instrument passed through the mouth or anus. Alternatively, the biopsy of organs deep within the body may require an operating room procedure.
Bleach: Chlorine bleach is highly effective in killing HIV within minutes. It is available at most grocery stores and is commonly recommended for killing any virus or other microbe that may be in such body fluids as blood, saliva, and stool. Bleach is usually diluted 1:10, or one part of bleach in ten parts of water. This dilution can be applied to surfaces or in the washing machine for clothes.
Blood count: Blood is composed of red blood cells (erythrocytes) which carry oxygen to all parts of the body, white blood cells (leukocytes) which help make up the immune system, and platelets (or thrombocytes) which are required for blood clotting. All three kinds of cells can be counted under a microscope. A low red blood cell count is called anemia (see above); a low white blood cell count is called leukopenia (see below), and a low platelet count is called
thrombocytopenia (see below). People with HIV infection commonly have low red counts, low white counts, and low platelet counts. A blood count is a routine procedure for clinical laboratories; it is a relatively simple, inexpensive, and standard test to evaluate people with HIV infection.
Bone marrow: Bone marrow is the tissue in the central portion of many bones where blood is manufactured. Bone marrow can be withdrawn (by placing a needle in the hip bone) and analyzed to detect abnormalities in the production of red blood cells, white blood cells, or platelets.
Buyers’ clubs: Buyers’ clubs are groups, or even individuals, that make underground drugs (drugs not approved by the Food and Drug Administration and thus not available as licensed drugs in the United States) available to people with HIV infection. The advantage of a buyers’ club is that it offers the person with HIV infection the opportunity to take drugs that might require long periods of testing before becoming widely available. The disadvantage is that the drugs are untested and could be toxic or useless.
Candidiasis: Candidiasis is an infection caused by the fungus Candida albicans. People with HIV infection commonly have candidiasis in the mouth (thrush), in the esophagus (candidal esophagitis), or in the vagina (vaginal candidiasis). The diagnosis can be confirmed by microscopic examination of the patches. Candidiasis is common in people who do not have HIV infection. In people who do have HIV infection, candidiasis is especially common, sometimes severe, and likely to recur. Treatment with nystatin, clotrimazole, or ketoconazole—all taken by mouth—is usually successful. When the infection is extensive or when it involves deeper organs, it is sometimes necessary to treat with intravenous amphotericin B.
CD4 cells: The blood contains several kinds of white cells, each of which plays a specific role in the immune system. CD4 cells (other names are T4 cells and T-helper cells) are the cells that HIV selectively infects. The number of CD4 cells frequently indicates the stage of HIV infection. Healthy people without HIV infection usually have around 1,000 CD4 cells in every milliliter of blood; counts of 400-700 are considered abnormally low, but not alarming. People with AIDS usually have counts of less than 200. Counts of less than 200 do not indicate AIDS, but suggest severe weakening of the immune system. The CD4 count is a relatively expensive test, but it is an important way of monitoring the state of the immune system. In any one person, however, the count varies considerably: the same laboratory performing the test on the same specimen can show counts that vary by as much as 20 percent. The CD4 count is also influenced by a variety of other medical conditions independent of HIV infection. As a result, although the CD4 count is frequently used to assess progressive disease, changes in the count are sometimes difficult to interpret.
Centers for Disease Control {CDC): The Centers for Disease Control is a federally funded institution located in Atlanta, Georgia. It has three responsibilities: to serve as an epidemiologic and public health resource for state and local health departments; to investigate epidemics; and to keep track of contagious diseases and other diseases important to public health. The CDC has about 4,000 employees, including 800 physicians and Ph.D.’s. In the past, the CDC has been responsible for much of what we know about Lyme disease, Legionnaires’ disease, and toxic shock syndrome. More to the point, the CDC provided much of the early epidemiologic data that identified the symptoms of AIDS, the kinds of behavior that risked AIDS, and how AIDS was transmitted—in fact, the CDC was responsible for the name AIDS. At present, the CDC is the storehouse for all reported cases of AIDS in the United States. It provides guidelines for disease prevention and gives advice on safety for health care providers. It is responsible for funding state and local agencies that test for HIV, counsel, and collect data.
Chloral hydrate: Chloral hydrate is a sedative used to treat insomnia. It is usually taken fifteen to thirteen minutes before bedtime. Using chloral hydrate regularly for more than two weeks often reduces its effectiveness. Major side effects include stomach irritation, residual sedation, or a hangover. Chloral hydrate should be used with great caution in people who are depressed, who may commit suicide, or who have a history of drug abuse.
Clostridium difficile: People who take antibiotics often develop diarrhea as a side effect. A relatively common and particularly severe cause of this diarrhea is a microbe called Clostridium difficile. Almost any antibiotic can cause this complication, but the most frequent causes are ampicillin, amoxicillin, clindamycin, and a group of drugs called cephalosporins that includes cefixime (or Suprex), cefuroxime, cephalexin (or Keflex), and cefaclor (or Ceclor). People who have diarrhea while taking these or any other antibiotics should stop taking the antibiotics and call their physicians. A test of stool will determine if Clostridium difficile is the cause. If it is, it can be treated with metronidazole or vancomycin hydrochloride. Vancomycin is preferred for serious cases of diarrhea, but it costs about $200 to $300. Metronidazole is less expensive—$10 to $20—and equally effective unless the person has severe colitis.
CMV: CMV, which is short for cytomegalovirus, is a virus commonly found in people without HIV infection. Usually the immune system holds CMV in check, and it remains dormant in the body without causing any serious disease. With a severely weakened immune system, however, CMV may cause serious infection. The site of the infection can be in the eye, lung, liver, gastrointestinal tract, bone marrow, brain, or widespread in many of these areas. The virus can be detected by cultures of blood, cultures of urine, or biopsies of any of the organs that are affected. CMV is difficult to treat: some antibiotics seem to inhibit its growth in the test tube, but no one agrees about when these drugs should be used. An exception is CMV of the retina in the eye: CMV retinitis may result in loss of vision and requires treatment with a drug which slows the progression of the infection and which must be taken indefinitely.
Co-factor: A co-factor is anything—microbes, proteins, hormones, genes—which makes a disease progress more rapidly. With HIV infection, co-factors are only suspected but may include other viruses (like cytomegalovirus), age, genetic resistance or predisposition, and certain hormone-like substances, called cytokines, released by lymphocytes.
Colon: The gastrointestinal tract—which starts at the mouth and ends at the rectum—includes the esophagus, stomach, small intestine, colon, and rectum. The colon and the small intestine are commonly the sites of infections that cause diarrhea. To diagnose problems in the colon, common procedures are colonoscopy and sigmoidoscopy. These procedures permit visualization and biopsy of the colon by passing a tube through the rectum.
Community-based organizations (CBOs): Community-based organizations are also called AIDS-advocacy organizations and AIDS service organizations (ASOs). They are organizations and agencies that provide services to people with HIV infection, as well as education and prevention programs for the whole community. The leaders of community-based organizations are lay people, ordinary people who do not come from the government or from organized medicine—although many community-based organizations have physicians as advisers, and most receive public funds.
Examples of community-based organizations dealing with other diseases are the American Lung Association, the American Heart Association, and the American Cancer Society. There is no similar nationwide organization for people with HIV infection or AIDS. Nevertheless, most cities have one or sometimes several such organizations: examples include Shanti in San Francisco, the Gay Men’s Health Clinic in New York City, and HERO in Baltimore. The types of services offered vary but may include counseling, crisis support, financial assistance, transportation, meals, housing, support groups, legal aid, social services, education, psychological support, hotlines, buyers’ clubs, and medical services. Most of these organizations have a paid professional staff but rely heavily on volunteers. Funding usually comes from state governments, corporations, foundations, and local fundraising events.
Computerized tomography scan (CATscan): CAT scans are a particular kind of x-ray that provide a three-dimensional view of the body. Conventional x-ray tests provide a
two-dimensional view of the body; CAT scans use computers to stack a series of
two-dimensional x-rays together to form a three-dimensional image of the body. CAT scans can be done of the entire body or of parts of it. The person receiving a CAT scan first receives an injection of what is called contrast material—material that shows up under x-rays. Some people have allergic reactions to contrast materials and should not receive them again. The person receiving the CAT scan is next put into a chamber with a scanner that circulates around the body, producing three-dimensional images in parallel sections of about an inch or less. CAT scans, first developed in the 1970s, are an excellent method for detecting tumors, infections, or other changes in the anatomy of the brain, chest, abdomen, or other parts of the body. They are also expensive, usually costing around $300 to $800.
Constitutional symptoms: Symptoms caused by the impact of an illness on the entire body or constitution are frequently referred to as constitutional symptoms. Included are fatigue, achiness, weight loss, fever, and night sweats. Constitutional symptoms are present in many types of infectious diseases, tumors, and other medical conditions ranging from the serious to the trivial. For people with HIV infection, constitutional symptoms may be a result of HIV infection itself or the result of such opportunistic illnesses as Pneumocystis pneumonia, tuberculosis, or widespread CMV infection.
Contagious: A disease that is contagious can be passed from one person to another. A disease that is infectious is caused by a microbe. All diseases that are contagious are also infectious; but some diseases, like toxic shock syndrome, are infectious and not contagious. HIV is both infectious and contagious, but is contagious only with specific types of contact.
Corticosteroids (also known as steroids, glucocorticosteroids, prednisone, and cortisone): Corticosteroids are drugs used to reduce the immune response. Numerous preparations are available that can be taken intravenously, by mouth, or in an ointment applied to the skin. Using high doses of corticosteroids for a long time can be dangerous: they reduce the immune system’s defenses against certain infections. Corticosteroids are sometimes considered especially dangerous for people with HIV infection, whose immune defenses are already weakened. Nevertheless, many of the complications of HIV infection appear to result from an overly abundant but misdirected immune response. As a result, these complications of HIV infection respond well to corticosteroids, though the drug should be taken at the lowest doses for the shortest period.
Cryptococcosis: Cryptococcosis is an infection caused by the fungus Cryptococcus neoformans. This fungus can cause infection in otherwise healthy people. In people with HIV infection, however, it is especially severe, frequently causing meningitis. Common symptoms include headache, fevers, vision problems, and seizures. The diagnosis is usually made by analyzing cerebrospinal fluid obtained with a spinal tap (see below). The disease is treated with amphotericin B given by vein or fluconazole given by mouth; when treatment is stopped, the disease tends to recur so that long-term treatment is generally necessary.
Cryptosporidiosis: Cryptosporidia are parasites that infect the intestine and cause diarrhea. This infection, called cryptosporidiosis, can occur in otherwise healthy persons, but the diarrhea generally does not last long and is not severe. Cryptosporidiosis in people with HIV infection often causes devastating diarrhea that persists for months. People with cryptosporidiosis may lose large amounts of fluid and nutrients and, consequently, become severely malnourished. The diagnosis is usually established by simply examining the stool under a microscope to detect the parasite. There is no universally accepted form of treatment except to replace the lost fluids and nutrients.
Culture: A culture, in medical terms, is a medium in which microbes can grow. HIV is grown in cultures containing lymphocytes. If a sample of a person’s blood is put into such a culture, and HIV grows, the person is infected with HIV. Other blood tests for HIV are polymerase chain reaction (see below) or the P24 antigen test (see below). The usual blood test for HIV detects antibodies to the virus instead of the virus itself. The antibody test is usually preferred because it is less expensive, better standardized, and more readily available.
Cytomegalovirus: See CMV, above.
Dantrolene (or Dantrium): Dantrolene is one of several muscle relaxants. The most common side effect is muscle weakness that usually disappears after taking the drug for several days. Other side effects include hepatitis (see below), diarrhea, gastric intolerance, depression, insomnia, and frequent urination.
Dementia: See AIDS dementia complex, above.
Dextroamphetamine sulfate (or Dexedrine): Dextroamphetamine sulfate, along with methylphenidate hydrochloride (or Ritalin Hydrochloride), stimulates the brain. It is usually given to people with HIV infection with AIDS dementia complex to counter the symptoms of apathy and social withdrawal. The most common side effects are nervousness and insomnia. Both side effects can usually be controlled by decreasing the dose and by avoiding taking the drug late in the day.
Dormant: See Latency, below.
Dysphagia: Dysphagia means difficulty with swallowing. The most common cause of dysphagia is an infection by Candida albicans, a fungus that can be easily treated (see Candidiasis, above). Less frequent causes are infections with herpes or CMV. In some people dysphagia has no readily apparent cause. The usual method of finding the cause of dysphagia is endoscopy, a procedure in which a tube is placed in the esophagus to visualize and biopsy the lesions. X-ray examinations are another means of viewing the esophagus. In many cases, neither of these tests is done, and the person is presumed to have a Candida infection if he or she also has thrush (see below) and if swallowing is painful.
ELISA test: The ELISA (pronounced eelissa) is a blood test done to detect antibodies to certain microbes, among which is HIV. The ELISA is the first of two standard tests done together to detect antibodies to HIV. The test is extremely sensitive but not very specific. Sensitivity means that the test is able to detect HIV infection; specificity means that the test specifically detects a particular infection and no other. In other words, with ELISA, people who have HIV infection will rarely have a falsely negative test, but people who do not have HIV infection will commonly have a falsely positive test. As a result, the ELISA is used as a screening test, and those who are positive have a second test on the same blood sample called a Western blot.
The Western blot test, combined with an ELISA, is over 99 percent accurate in both sensitivity and specificity. The combination of tests is generally offered free of charge from most health departments and at a cost of $20 to $150 from commercial laboratories. The test offered may be anonymous, meaning that the person receiving the test cannot be identified, or it is confidential, meaning that privacy is honored but a record is kept identifying a specific person with the test result. The ELISA is easily performed, but the Western blot is more complicated and often done only by reference laboratories or on certain days of the week. For this reason, the results may not be available for several days or even weeks. The test results are usually either positive or negative, but occasionally people have Western blots that can not be clearly interpreted and the test results are considered indeterminant. The usual recommendation for
people with indeterminant results is to have the test repeated in two or three months. People at a low risk for HIV and with indeterminant results almost never have HIV infection, and the cause of the indeterminant results is not known.
Encephalitis: Encephalitis is an infection of the brain. (Meningitis, by contrast, is an infection of the meninges, the membrane surrounding the brain and spinal cord—see Meningitis, below.) Encephalitis commonly causes headaches, fever, seizures, and neurologic problems. The diagnosis is frequently made on the basis of the person’s symptoms, combined with procedures to examine the brain such as computerized tomography scan [CAT scan) (see above); magnetic resonance imaging (MRI) (see below); or electroencephalogram (EEG). Diagnosis can also be made by analyzing the cerebrospinal fluid obtained by a spinal tap (see below). In people with HIV infection, the usual causes of encephalitis are infection with HIV itself or such opportunistic illnesses as toxoplasmosis or lymphoma.
Endoscopy: Endoscopy is a diagnostic procedure in which an instrument is passed through the mouth or rectum to examine an internal organ or to obtain a biopsy (see above). In people with HIV infection, the most common types of endoscopy are bronchoscopy to examine the lungs and endoscopies to examine the digestive system. Upper endoscopy of the intestine involves passing an endoscope through the mouth to examine the esophagus, stomach, or upper small intestine. Lower endoscopy of the intestine involves passing an endoscope through the rectum to examine the large intestine or colon. Endoscopes are flexible and can turn corners. Endoscopy requires the expertise of a specialist, can be done on an outpatient basis, and usually costs $1,200 to $1,800.
Enteritis: Enteritis is an inflammation of the small intestine; the most common symptom is diarrhea. In people with HIV infection, the microbes that usually cause enteritis are Cryptosporidia, microsporidia, Mycobacterium avium-intracellulare, and CMV. These microbes can be detected by examining stools under a microscope or with a biopsy of the small intestine done with an endoscope (see above, under Endoscopy), a tube that is placed through the mouth and into the small intestine.
Epidemic: An epidemic is a disease that occurs in many more people than would be expected during a given time. Epidemiology is the study of the factors that determine the frequency and distribution of diseases.
Fluconazole (or Diflucan): Fluconazole is used to treat fungal infections, primarily those caused by Candida albicans (thrush or candidal esophagitis) and Cryptococcus neoformans (cryptococcal meningitis). Fluconazole can be taken by mouth or by vein. Side effects are unusual; occasional problems are nausea, rash, or hepatitis.
Ganciclovir: Ganciclovir is used to treat infections caused by cytomegalovirus and occasionally for infections caused by herpes simplex and other viruses. It is given only intravenously. The most important side effect is a low blood count, especially neutropenia, which predisposes the person to bacterial infections (see Blood count, above, and neutropenia, below, under Leukopenia). If neutropenia is severe enough, the dose of the drug should be reduced, or the drug should be temporarily stopped.
Hemophilia: A person with hemophilia lacks a protein that helps the blood to clot. Hemophiliacs bleed easily, even with a trivial cut; many have severe hemorrhaging into the joints and eventually get joint disease. Hemophilia is inherited, and only by men; the gene for hemophilia is carried by women, who do not get the disease but who can pass the gene on to their sons.
Hemophilia has two forms, hemophilia A and hemophilia B; each form lacks a different clotting protein, called a clotting factor. Hemophilia is treated by substituting a commercial clotting factor for the clotting factor the blood lacks. The commercial clotting factor is extracted chemically from blood donated by hundreds or thousands of people. As a result, hemophiliacs are exposed to the blood of thousands of donors. Between 1978 and 1985, from the time HIV was introduced into the United States until the time the blood banks screened for HIV, hemophiliacs had a high risk of being infected with HIV. Approximately 70 percent of men with hemophilia A and 30 percent of men with hemophilia B acquired HIV infection from infected commercial clotting factors.
Since 1985, the risk of being exposed to HIV through clotting factors has dropped to practically nil. One reason is that donated blood is now screened for HIV; another reason is that clotting factors are heated and purified by detergents and biochemicals which kill HIV. The Centers for Disease Control found that between 1985 and 1988, only 18 hemophiliacs acquired    HIV, an annual rate of under one per thousand.
Hepatitis: Hepatitis is an inflammation of the liver. Many people have no symptoms and are unaware of having hepatitis. The symptoms, when people do have them, are loss of appetite, vomiting, yellow discoloration of the skin and eyes (jaundice), dark urine, sore stomach, and fever. Hepatitis is usually caused by a virus called hepatitis B virus (see below) that may be transmitted by sexual contact or blood-to-blood transmission. Since these are the same mechanisms of transmitting HIV infection, the same people who are likely to be infected with HIV infection are also likely to be infected with the hepatitis B virus. People with HIV infection are also prone to hepatitis caused by CMV and Mycobacterium avium-intracellulare. Alcohol and drugs, including AZT, pentamidine, ketoconazole, trimethoprim-sulfamethoxazole, and INH, may also cause liver inflammation. The diagnosis of hepatitis is easily made with blood tests to determine liver function and to detect specific microbes, including hepatitis B, hepatitis C, and hepatitis A viruses. When the cause is unclear, it is sometimes helpful to obtain a biopsy of the liver or to do tests of the gall bladder.
Hepatitis B virus: The hepatitis B virus is one of the microbes that causes hepatitis. Hepatitis B infection may be acute and cause serious symptoms that last up to a few weeks (see above, under Hepatitis); it may be chronic with occasional symptoms and abnormal liver tests that last for months or years; or it may cause no symptoms at all and may only show up on a blood test. About 5-10 percent of people with hepatitis B infection become chronic carriers of hepatitis B virus; they continue to carry the virus and can transmit it to others for years. The hepatitis B virus is transmitted the same way HIV is, by sexual contact or blood-to-blood transmission.      Hepatitis B is transmitted far more efficiently than HIV, so that a person exposed by a needlestick accident involving a person with both infections is about twenty times more likely to develop infection with the hepatitis virus. The blood supply used for transfusions is screened for the hepatitis B virus and is therefore an unlikely source of this infection.
There is no evidence that hepatitis B is any different in people with HIV infection than in people without HIV infection. However, the presence of liver damage or ongoing inflammation may complicate the use of certain drugs that (like AZT) require the liver for metabolism or that (like AZT and pentamidine) may occasionally cause further liver damage (see Hepatitis, above). Once infection takes place, no treatment will eradicate the hepatitis virus. Infection may be prevented, however, by a vaccine. The vaccine is recommended for the people at risk for this infection: people who share needles to inject drugs, people who practice unsafe sex with gay men, family members who live in the same household, sex partners of people known to be hepatitis B carriers, and health care workers. Three injections are required, at a cost of about $100 for all three doses.
Herpes simplex virus: Herpes simplex is a virus that commonly causes infections of the mouth and genitals. There are actually two different viruses: though similar in many respects, one kind seems mostly likely to infect the mouth and the other, the genitals. The symptoms of both infections are blisters on the mouth or genital area that first contain clear fluid, then become filled with pus, finally form scabs, and eventually disappear. Herpes simplex is a persistent virus: the virus remains dormant most of the time and then causes recurrent symptoms intermittently over a period of years. The initial infection with herpes simplex virus is often severe with large areas of blisters, occasional fevers, and pain and tingling in the area involved. Subsequent attacks are usually milder. The virus is transmitted to others by contact with the mouth or genitals, especially when the blisters are present.
Both the oral and the genital form of herpes are common infections in the general population; in people with HIV infection, however, the blisters are likely to be more severe, be spread over relatively larger areas, and, most importantly, persist for longer periods of time. In a person with HIV infection, herpes blisters that persist over one month constitute an AIDS-defining diagnosis. Treatment with a drug called acyclovir usually heals the blisters, prevents recurrences, and reduces the risk of transmitting the virus to others. Acyclovir is available as an ointment to put on the blisters and as tablets to be taken by mouth. In people with AIDS who have severe herpes infections, acyclovir is also given intravenously; once the infection is under control, the tablets are often given for extended periods to prevent recurrences.
Herpes zoster: Herpes zoster is caused by the same virus that causes chickenpox. The virus persists in the body and may cause symptoms decades after the original infection. Attacks after the first infection are called shingles, or herpes zoster. The skin sores with herpes zoster are similar to those of chickenpox and those of herpes simplex. The sores begin as red spots that become blisters filled with water; the blisters break down into sores with pus, finally scab over, and eventually disappear. Unlike herpes simplex infections or chicken-pox, however, the later recurrences of herpes zoster are usually restricted to the area served by a single nerve. In other words, the blisters are restricted to one side of the body, usually in a band across the face, chest, abdomen, back, or leg.
In many people, recurrences of herpes zoster are accompanied by post-herpetic neuralgia, a pain at the site of blisters that may persist for months after the blisters are gone. Post-herpetic neuralgia is fortunately infrequent among people with HIV infection. Herpes zoster is more common and more severe in people with HIV infection. It does not, however, necessarily mean that the immune system is weakening, and it clearly does not indicate AIDS. The diagnosis is generally made with a microscopic examination and culture of blisters, but the appearance of the blisters is usually all a physician needs to make a diagnosis. Acyclovir appears to hasten healing, but high doses of the drug must be given by mouth or by vein.
Hickman catheter: People who require long courses of drugs given by vein will often have a tubing called a Hickman catheter. The catheter is inserted by a specialist, usually a surgeon, through the skin of the chest, and then tunneled under the skin to a vein in the chest. The end of the catheter comes out the chest wall above the breast. Drugs can be injected into the catheter as necessary. The advantage of a Hickman catheter is that it permits access to the vein without
repeated needlesticks in the arms. Similar devices are also available; the Hickman catheter is simply one of the most frequently used.
It is important to know that the area around any catheter in a vein can become infected. Symptoms of infection of the area where the catheter comes out are redness and pain, and sometimes pus. Symptoms of infection around the catheter inside the body are fever
and chills. Anyone with a Hickman catheter and these symptoms should tell a physician right away. Antibiotics should be given immediately, and sometimes the catheter needs to be removed.
HIV: HIV stands for the human immunodeficiency virus. HIV has had several names. It was first called lymphadenopathv associated virus (LAV) by Luc Montanier in France in 1983, and next called human T-lymphotropic virus III (HTLV-III) by Robert Gallo in the United States in 1984. HIV is now the official international name of this virus. HIV is the virus responsible for AIDS. There are occasional arguments that perhaps HIV does not cause AIDS or is responsible for AIDS only in combination with other viruses, but at present, the great majority of scientific authorities accept HIV as the sole cause of AIDS. The only other causes are the closely related viruses such as HIV-II that are responsible for a very few cases of AIDS, primarily in Africa.
Idiopathic thrombocytopenic purpura (ITP): See both Platelets and Thrombocytopenia, below.
Immune system: The human body is defended against a multitude of microbes by a complex system called the immune system. The principal components of the immune system are cells called B lymphocytes, neutrophils, and T lymphocytes. B lymphocytes make antibodies, the proteins that attack bacteria and viruses; neutrophils envelop and kill bacteria; and T lymphocytes provide communication between the parts of the immune system. Although these three components are somewhat interdependent, each takes primary responsibility for defense against certain types of microbes. For this reason, people deficient in different components are prone to infections with quite different microbes.
The cell type that is primarily affected in people with HIV infection is a type of T lymphocyte called a CD4 cell (see above). The most common infections encountered in people with few CD4 cells are caused by Pneumocystis carinii, cytomegalovirus, Mycobacterium avium-intracellulare, herpes simplex virus, herpes zoster, Candida albicans, Toxoplasma gondii, Cryptosporidium, Cryptococcus, Salmonella, and the bacterium that causes tuberculosis. People with immune systems weakened by HIV are not only subject to high rates of infections with these organisms, but the infections tend to be severe, prolonged, and recurrent. At the same time, many other microbes that commonly cause infections in everyone do not appear to be unusually common or severe in people with HIV, presumably because the other components of the immune defenses remain relatively strong.
Incubation period: The incubation period of a disease is the time interval between infection with a microbe and the first symptoms of disease. For influenza and common colds, the incubation period is usually several days; for measles, chickenpox, mumps, and infections caused by many other viruses, the incubation period is two to three weeks. An unusual feature of HIV infection is that the first symptoms of a weakened immune system usually do not occur until several years after the infection takes place.
Infectious: See Contagious, above.
Influenza vaccine: The influenza vaccine varies in effectiveness, depending on whether the strain of virus in the vaccine is related to the virus which is causing the influenza. The effectiveness of the vaccine changes every year. In most years, however, the vaccine probably prevents about 70 percent of the cases of influenza, and those who become infected despite having been vaccinated usually have less severe symptoms. Influenza does not seem to be unusually common or severe in people with HIV infection. The only problem specific to people with HIV infection is that the symptoms of influenza can be confused with the symptoms of other pneumonias such as
pneumocystis pneumonia (see Pneumocystis carinii, below), a confusion it would be nice to avoid. Therefore, the CDC’s Advisory Committee on Immunization Practices recommends that people with HIV infection routinely get the influenza vaccine every year.
Informed consent: Informed consent is a form of protection for people considering taking an HIV antibody test or undergoing certain medical procedures (like an operation) or considering participation in a clinical trial. Before taking the test, undergoing the procedure, or participating in the trial, the person or the person’s representative must sign an informed consent form stating that Tie or she has been informed about the purpose, benefits, risks, and alternatives to the test, procedure, or trial, and that he or she consents to it. In the case of participation in a clinical trial, the informed consent form explains the purpose of the trial, what will be done, the risks of
participation, the benefits of participation, what other treatments are available, and the right of the participant to leave the trial at any time.
Inoculum size: Inoculum size is a term used in the field of infectious diseases to describe the number of microbes necessary to cause an infection. In HIV infection, for example, a certain number of viruses is required before infection takes place. The specific number is not known. What is known is that the probability of transmitting HIV with the transfusion of one unit (or 500 milliliters) of infected blood is 80 to 90 percent. The probability of transmitting HIV with a
needlestick injury, which injects only a fraction of a milliliter of blood, is 0.4 percent. This difference in the probabilities of transmission is most likely due to differences in inoculum size.
Interferons: Interferons are proteins that cause cells to resist attack by certain viruses. Interferons are usually produced by the body, but they are also made artificially and used as medications. The major side effects of interferons are the achiness and fever that accompany flu: it is the interferon produced by the body that causes these symptoms during flu.
Isoniazid (INH): Isoniazid is the standard drug used to treat and prevent tuberculosis. Isoniazid is usually recommended for any person with HIV infection who has tuberculosis or who has a positive tuberculosis skin test. The usual dose is 300 milligrams, taken by mouth. The most important side effect is hepatitis, including jaundice (yellowish skin and eyes), dark urine, nausea, and abdominal pain. This side effect is more likely in people who already have liver damage for other reasons, and in older people. People taking isoniazid and having these symptoms should stop taking the drug immediately and call their physician.
Kaposi’s sarcoma: Kaposi’s (pronounced kaposhee’s) sarcoma is a tumor of blood vessels. Next to Pneumocystis pneumonia, it is most likely to be the first AIDS-defining diagnosis that people have; approximately 20 percent of all people with AIDS have Kaposi’s sarcoma. The symptoms of Kaposi’s sarcoma are purplish nodules, a quarter of an inch to an inch in diameter, anywhere on the skin. The nodules will grow in size and number. They sometimes occur on
internal organs like the lung, brain, and gastrointestinal tract, though they often cause no specific symptoms at these sites. Some nodules are painful. The face and legs may swell if the lymph channels nearby are blocked. If Kaposi’s sarcoma becomes extensive, people may have fever, weight loss, and severe fatigue.
The diagnosis can be established by a biopsy of the nodules. Biopsies are easy to do with nodules on the skin, but more difficult when the nodules are on internal organs. The main reason to do the biopsy is that the nodules might possibly turn out to be something other than Kaposi’s sarcoma; and if they are Kaposi’s sarcoma, they are an AIDS-defining diagnosis. Therapy is controversial: Kaposi’s sarcoma is rarely life-threatening, and treatment is neither easy nor universally effective. If the nodules are painful, disfiguring, or complicated by swelling, they can be treated with radiation or interferon; if the nodules have spread widely over the skin or into internal organs, causing symptoms, they can be treated with the same drugs given to people with cancer.
Ketoconazole: Ketoconazole is a drug given by mouth for infections caused by Candida albicans and other fungi. Ketoconazole requires acid in the stomach to be absorbed into the system. People should therefore not take other medicines that neutralize stomach acids until at least two hours after the dose of ketoconazole. Side effects include nausea, vomiting, hormonal problems (menstrual problems and reduced sex drive), rash, headaches, and liver damage.
Latency: Latency and dormancy (which literally means sleeping) mean the same thing: a microbe is in the body but is not actively reproducing, not invading any tissues, and not causing symptoms. Examples of microbes that are latent or dormant in many or most healthy people are: Pneumocystis carinii, Toxoplasma gondii, herpes simplex virus, the virus that causes herpes zoster, and cytomegalovirus. Once in the body, these microbes remain in the body. They remain latent or dormant until something tilts the balance in the immune system and permits them to become active.
Leukopenia: Leukopenia means a low number (or penia) of white blood cells (or leukocytes—leuko means white), the cells of the immune system that fight infection. Leukocytes include lymphocytes (cells that recognize foreign material) and neutrophils (cells that gobble up microbes). The normal leukocyte count is 4,000 to 8,000 per milliliter of blood. In people with certain infections, especially with bacterial infections, the leukocyte count is high (leukocytosis). In people with viral infections, including HIV infection, the leukocyte count is low (leukopenia). Having a low count of lymphocytes is called lymphopenia; lymphopenia is the expected result of HIV infection. A low count of neutrophils is called neutropenia; neutropenia can be caused by HIV itself or by some of the drugs commonly taken during HIV infection. Neutropenia becomes worrisome if the count is less than 750 per milliliter; if the count is less than 500 per milliliter, the person is prone to bacterial infections.
Lumbar puncture: See Spinal tap, below.
Lymph glands: The lymphatic system is a widespread network, like the blood circulation system, of channels that carry lymph. Lymph is a clear fluid containing lymphocytes, or white blood cells (including CD4 cells), that are a part of the immune system. Lymph is manufactured in the lymph glands, which are clumps of lymphatic tissue distributed widely throughout the body. When lymph glands are near the surface of the skin, they can be felt as bumps below the skin’s surface. The usual locations where they can be felt are the back of the neck, below the jaw, under the armpits, and in the groin. Lymph glands are commonly swollen and sometimes painful and tender when they are infected. Many infections involve the lymph nodes. In HIV infection, swollen lymph glands are likely to occur in three different circumstances: with persistent generalized lymph adenopathy, or PGL (see below), in which many lymph glands are swollen for months; with infection of the lymph glands by certain opportunistic diseases; and with lymphomas, which are tumors of the lymphatic system seen more frequently in people with HIV infection than in the general population. Swollen lymph glands may require diagnostic tests: the usual is a biopsy of the lymph gland or removal of the whole gland to permit microscopic examination of the lymphatic tissue.
Lymph adenopathy: Lymphadenopathy means swollen lymph glands. Swollen lymph glands are most common at the back of the neck, along the jaw, in the armpits, and in the groin. The lymph glands may feel like rubbery, discrete nodules that are rarely tender to touch and often pea-sized; glands of this description are common in everyone and in several conditions unrelated to HIV infection. If they are swollen to abnormal size for longer than a month in at least two different areas, they constitute persistent generalized lymphadenopathy (PGL) (see below).
Lymphoma: Lymphoma is a cancer of the lymphatic system. Lymphoma occurs most frequently in people without HIV infection, but people with weakened immune systems, including those with HIV infection, have lymphomas about forty times more frequently than
normal. About 1 to 3 percent of people with AIDS have lymphomas, and for people with AIDS, lymphomas are classified as opportunistic tumors. There are many types of lymphomas: some progress extremely slowly, cause few symptoms, and require minimal treatment; some are more severe. People with AIDS generally have lymphomas called non-Hodgkin’s lymphomas of B cell origin. These lymphomas tend to be severe, and they also tend to involve unusual areas of the body like the brain, liver, kidneys, intestines, and lungs. The diagnosis is usually established with a biopsy. Treatment is variable and often requires the assistance of a specialist in cancer treatment using cancer chemotherapy or radiation treatment.
Magnetic resonance imaging (MRI): Magnetic resonance imaging is a technique used to make a three-dimensional image of the interior of the body. Though the technique is somewhat different from a CAT scan (see Computerized tomography scan, above), the images are similar. The person getting an MRI is placed inside a large tubular structure and remains motionless for thirty to sixty minutes: the worst problems are boredom and claustrophobia. During that time, the person’s body is bathed in a magnetic field, which causes the atoms in different tissues to give off tiny radio signals. The signals are different depending on the kind of tissue. An MRI is better than a CAT scan at detecting diseases of the brain and spinal cord. MRI is painless, harmless, and does not involve exposure to radiation; the body is not exposed to any kind of potentially harmful radiation. MRIs are also expensive, from $500 to $1,000.
Megestrol Acetate (Megace): Megestrol is a drug that stimulates the appetite. The drug has virtually no serious side effects, even with doses as high as 800 milligrams daily.
Meningitis: Meningitis is an infection of the meninges, the membrane that envelopes the brain and spinal cord. The most common cause of meningitis in people with HIV infection is Cryptococcus (see under Cryptococcosis, above).
Meprobatnate (e.g, Equanil, Miltown): Meprobamate is a drug that acts on the central nervous system much as barbiturates do. Meprobamate is used most commonly to treat anxiety. As with barbiturates, meprobamate can cause drowsiness, lethargy, and lack of coordination. Continued use for weeks or months may cause tolerance—that is, increasing doses are required for the same effect. Continued use may also cause psychological and physical dependence. Withdrawing the drug suddenly after prolonged, regular use may cause severe reactions. Other side effects can include stomach irritation and allergic reactions.
Methadone hydrochloride: Methadone is an opiate that is commonly used to control narcotic withdrawal symptoms and to maintain people addicted to morphine-like drugs, particularly heroin. Methadone maintenance is permitted only in programs approved by the Food and Drug Administration and the designated state authority.
Methadone can be given by mouth or by vein. Side effects are those shared by all morphine-like drugs that depress the central nervous system: dizziness, mental clouding, depression, and sedation. Methadone may cause physical dependence. If it is stopped abruptly after prolonged and regular use, it can cause withdrawal symptoms.
Metronidazole (e.g., Flagyl): Metronidazole is an antibiotic taken by people with HIV infection for common intestinal infections and common dental problems like gingivitis (inflammation of the gums) and periodontitis (infection of the structures that support the teeth). The drug is given by mouth or by vein. Side effects are unusual, primarily nausea and stomach pain. The side effects can improve if the drug is taken with meals or if the dose is reduced. Taking this drug for periods of months may cause pain in the feet that resembles the pain of HIV neuropathy (see below). This pain usually goes away when the drug is stopped.
Microbes: Microbes are organisms so small they require a microscope to be seen. They can be bacteria, viruses, parasites, or fungi. HIV is one example of a virus. Microbes cause infectious diseases. The microbes that commonly cause the opportunistic infections that accompany HIV infection are as follows:
Viruses: Cytomegalovirus, herpes simplex, herpes zoster, mol-luscum contagiosum
Bacteria: Mycobacterium avium-intracellulare, Mycobacterium tuberculosis (the cause of tuberculosis), Salmonella, Nocardia
Parasites: Toxoplasma gondii, Pneumocystis carinii, Cryptosporidium, Isospora
Fungi: Cryptococcus, Histoplasma, Candida albicans
Mycobacterium avium-intracellulare {MAI): MAI is related to the bacterium that causes tuberculosis, though it is not contagious and is more difficult to treat. In the late stages of HIV infection, infection with MAI is spread widely throughout many organs in the body. It can cause fever, pneumonia, diarrhea, hepatitis, and many other complications.
National Institutes of Health (NIH): The NIH is a federal organization located in Bethesda, Maryland, that funds scientific research. The NIH is the world’s largest research organization. With a budget of over $10 billion a year, the NIH is responsible for funding about a third of all research in the biomedical sciences, including research related to HIV infection, in the United States. Some of the research sponsored by NIH is intramural, that is, it is conducted by the
approximately one thousand researchers inside NIH; most of the research is extramural, at universities and medical schools throughout the country. Extramural research grants are awarded on the basis of priority, as determined by expert review of proposals. The NIH is divided into fifteen different institutes, each with a different scientific specialty: the National Institute for Allergy and Infectious Diseases (MAID) is responsible for most of the research into HIV
infection. The NIH is not related to the Centers for Disease Control (see above), except that both are federally funded agencies with somewhat different roles in combating HIV and other diseases.
Funding for research into HIV infection from sources other than NIH comes from other federal agencies (Department of Defense, National Science Foundation, Veterans Administration, and the Centers for Disease Control), pharmaceutical companies, local
governments, and private foundations. Funding for this research escalated rapidly in the late 1980s until the total HIV research budget exceeded the funding for heart disease research at a time when heart disease was responsible for twenty times more deaths than AIDS was. Some view this as inappropriate, given the relative impact of the two; others feel AIDS research is underfunded, given its importance as a public health problem and as a prototypic disease for many other conditions.
Neuropathy: Neuropathy is an illness involving the nerves. Nerves are responsible for (among other things) the movement of muscles and the sensation of touch, including the sensation of pain. The symptoms of a neuropathy can therefore be weakness of a muscle or pain and tingling. In people with HIV infection, the most frequent symptoms of neuropathy are painful feet and legs.
Opportunistic infections: In all infectious diseases, the body’s defenses are, for a while, inadequate to control microbial invasion. Many microbes can cause disease in people who are otherwise healthy. Other microbes, however, are fairly hapless and can cause disease only in people whose immune defenses are weakened. These microbes are called opportunistic microbes because the microbe takes the opportunity offered by a weakened immune system to cause disease. The opportunistic microbes that most frequently infect people with HIV infection are summarized under Microbes.
Opportunistic tumors: Opportunistic tumors, like opportunistic infection, occur primarily in people with weakened immune systems. In people with HIV infection, the major opportunistic tumors are Kaposi’s sarcoma and certain types of lymphoma.
Oral hairy leukoplakia (OHL): The symptoms of oral hairy leukoplakia are white (leuko) patches (plakia) on the tongue and elsewhere in the mouth. It usually produces no symptoms, but may distort taste or cause pain. It is caused by the same virus that causes infectious mononucleosis. These patches often appear similar to those of thrush; in fact, oral hairy leukoplakia is often diagnosed when people who appear to have thrush do not respond to the usual treatment. It can also be diagnosed with a biopsy of the patches. Oral hairy leukoplakia seems to occur exclusively in people with HIV infection. It generally indicates progressive weakening of the immune system; without treatment for HIV, the first AIDS-defining diagnosis is likely in the next two or three years. The treatment for oral hairy leukoplakia is an antiviral drug called acyclovir.
P24 antigen test: The P24 antigen test is, like the PCR test (see Polymerase chain reaction, below) and a culture (see above) for HIV, a test that detects the presence of HIV in the blood. P24 is one of the several proteins (the protein with a molecular weight of 24,000) that make up HIV. An antigen is anything that causes the immune system to identify it as foreign and to manufacture antibodies against it. A P24 antigen test detects P24 and therefore HIV. Unlike PCR and HIV cultures, however, the P24 antigen test is not especially sensitive, and most people with HIV infection have tests for the P24 antigen that are negative.
Levels of P24 are highest both early and late in the disease; the numbers of HIV are likewise highest at the same times. Some physicians therefore suggest that the P24 antigen test might help track the course of the disease in people with HIV infection. That is, it might identify people with HIV infection who are likely to develop symptoms and who are most likely to transmit the virus to others; and it might help evaluate how people are responding to antiviral drugs.
Pentamidine: Pentamidine is a drug used to treat or prevent pneumocystis pneumonia. It can be taken by vein or delivered directly into the lungs as an aerosol, using a breathing machine. When given by vein, pentamidine often has such side effects as low blood pressure (causing fainting), low blood sugar, high blood sugar (diabetes), kidney failure, liver disease, low blood counts, or inflammation of the pancreas. These side effects are common when the drug is given by vein. They are rare or don’t occur at all when pentamidine is taken as an aerosol, since so little of the drug gets into the system. Nevertheless, intravenous treatment is usually preferred for people who have advanced Pneumocystis pneumonia.
Persistent generalized lymph adenopathy (PGL): A diagnosis of PGL means that lymph glands are swollen for at least one month and at two different sites of the body, not counting the groin area. PGL often occurs early in HIV infection and is generally of little consequence unless the symptoms have actually been caused by another disease. (See Lymph glands and Lymphadenopathy, above.)
Platelets: Platelets are the component of blood that facilitates clotting. The number of platelets is often low in people with HIV infection— sometimes so extremely low that the person is prone to bleeding. The cause of the low platelets may be HIV infection itself, or it may be the drugs that are used to treat people with HIV infection.
Pneumococcal vaccine: The most common cause of bacterial pneumonia in people without HIV infection is a bacterium called Streptococcus pneumoniae or pneumococcus. Pneumococcus is also a common cause of pneumonia in people with HIV infection. Pneumococcal vaccine is recommended for people, including people with HIV infection, who are especially prone to frequent or severe infections by pneumococcus. It is best to take this vaccine relatively early in the course of the disease when the immune system is strong.
Pneumocystis carinii: Pneumocystis carinii is a parasite that commonly causes lung infection and pneumonia in people with HIV infection. Pneumocystis carinii pneumonia (PCP) is the most frequent serious opportunistic infection in people with HIV infection. When there is no explanation for immune suppression other than HIV infection, Pneumocystis pneumonia is an AIDS-defining diagnosis. The symptoms are cough without sputum, shortness of breath, and fever. These symptoms usually evolve over a period of several days or, more commonly, weeks. The diagnosis is generally established by a chest x-ray or studies of lung function, combined with a microscopic examination of respiratory secretions to show the parasite. Treatment with several drugs—most commonly trimethoprim-sulfamethoxazole and pentamidine—is successful. Treatment is most successful when started relatively early in the course of the infection.
Pneumonia: Pneumonia is an infection of the lungs. The usual symptoms are cough, fever, and shortness of breath. The causes of pneumonia vary, and the treatment depends on the cause.
Polymerase chain reaction (PCR): Polymerase chain reaction is a very sensitive test, developed in the late 1980s, for detecting tiny quan-      ’ tities of HIV (see Retrovirus, below). Unlike the standard blood test for HIV infection which detects antibodies to HIV, the PCR detects HIV itself. The test is very accurate; over 95 percent of people with HIV infection will test positive on a PCR test. The great majority of people who take the standard antibody test for HIV infection need not take the PCR test. PCR is most useful when the results of the antibody test are ambiguous and in research studies. PCR is similar to a culture (see above) for HIV but is substantially less expensive. Neither PCR nor cultures for HIV, however, are available in most laboratories.
Progressive multifocal leukoencephalopathy: Progressive multifocal leukoencephalopathy is a viral infection deep in the brain that is found only in people with severely weakened immune systems, including, occasionally, people with HIV infection. Progressive multifocal leukoencephalopathy has a distinctive appearance on CAT or MRI scans of the brain, but a diagnosis can be established definitely only with a biopsy. The infection tends to be progressive, and no therapy is known to be effective.
Prophylaxis: Prophylaxis is treatment to prevent a disease, as opposed to treatment to eliminate a disease already present.
PWA: PWA is the abbreviation for people with AIDS. The   PWA Coalition is one of several national organizations that provides newsletters, lists of resources, and research updates to people with HIV infection and AIDS.
Pyrimethamine {Daraprim): Pyrimethamine is an antibiotic used to treat toxoplasmosis (see below). The full treatment usually combines pyrimethamine with a sulfa drug like sulfadiazine or clindamycin. Pyrimethamine is taken by mouth. The major side effect after prolonged use is anemia. To avoid anemia, another drug, leucovorin, is given at the same time. Other side effects include gastric intolerance, allergic reactions, and hepatitis. Many of these reactions are the
result of the sulfa drug that is taken with pyrimethamine.
Research: See National Institutes of Health, above.
Retinitis: Retinitis means an inflammation (itis) of the retina, the layer of cells at the back of the eye that collects and send images to the brain. Retinitis usually causes some loss of vision. In people with HIV infection, the most common cause of retinitis is infection with cytomegalovirus (see above).
Retrovirus: Retroviruses are a type of virus. Retroviruses do not have DNA, the molecule that holds the code which cells use to reproduce themselves. Instead, retroviruses have RNA, a molecule that holds the retrovirus’s code, and an enzyme called reverse transcriptase, that turns RNA into DNA. When a retrovirus invades one of the cells of the body, it uses reverse transcriptase to turn its own RNA into DNA. This DNA then becomes part of the cell’s DNA, which in turn produces the retrovirus instead of the cell. Many different kinds of retroviruses infect many different kinds of animals. HIV is the most important retrovirus to infect humans; it causes disease in no other animal species.
Risk factor: A risk factor is a condition or behavior that makes it likely that a person with the risk factor will develop a condition—in this case, HIV infection. The major risk factors for HIV infection are needle-sharing with intravenous drug users and sexual contact with a person who has or may have been exposed to HIV. Another risk factor is having received blood products between 1978—when HIV infection was first known to exist in the United States—and May 1985, when the blood supply was first screened for HIV. Another risk factor is promiscuous or casual sexual contact without precautions. Still another is to be born to a woman with HIV infection: approximately 30 to 35 percent of women who are infected with HIV will pass the virus to the unborn infant. A minor risk factor is needlestick injuries in health care workers who care for people with HIV infection. Less than one percent of all people with HIV infection in the United States have no clearly defined risk factor, although many of these people either are too sick to provide adequate information or are providing information that is suspect.
Safer sex: Safer sex is a qualitative term. To be absolutely safe, sexual contact cannot involve an exchange of any body fluids—specifically, semen or vaginal secretions. The term safer sex recognizes the likelihood of human error and the inexactness of human knowledge. Safer sex refers to sexual intercourse using a condom and spermicide, or sexual practices that do not involve exchange of body fluids.
Seizure: A seizure is a convulsion, uncontrolled movements of the arms and legs accompanied by unconsciousness and inability to control urine or stool. The usual cause of seizures in people with HIV infection is an opportunistic infection or an opportunistic tumor of the brain, including toxoplasmic encephalitis, cryptococcal meningitis, or lymphoma. Less commonly, seizures are caused by an imbalance of electrolytes or by medications. Recurrent seizures can usually be controlled with drugs like Dilantin and phenobarbital. Anyone with recurrent seizures should be careful about his or her physical circumstances: be careful working on ladders, for instance, or driving. In many states it is illegal for a person with seizures to drive until seizures have been controlled for at least one year.
Seroconversion: The immune system usually takes several days or weeks to recognize a foreign substance like a virus and to produce antibodies to it. Six to twelve weeks (but sometimes years) after HIV enters the body, antibodies to HIV usually appear in the blood.
Physicians call the appearance of antibodies in the blood seroconversion. That is, the result of a test for antibodies in the blood serum converts from negative to positive.
Shingles: Synonymous with herpes zoster (see above).
Sinusitis: The sinuses are air sacs next to the passageway from the nose. Sinusitis is an infection of the sinuses, usually as a result of a cold or an allergy. Sinusitis is common in everyone, and is especially common in people with HIV infection. The reason for this is obscure.  Symptoms are pus drainage from the nose, headache, face pain, and fever. The usual treatment is with antibiotics taken by mouth, such as trimethoprim-sulfamethoxazole, amoxicillin, erythromycin, cephalexin (or Keflex), ciprofloxacin (Cipro), or tetracycline. Some people do not respond to these drugs, and their sinuses need to be drained, a procedure done by a specialist called an otolaryngologist (ear, nose, and throat specialist).
Spinal tap: A spinal tap, also called a lumbar puncture, is a procedure for obtaining cerebrospinal fluid, the fluid that surrounds the brain and the spinal cord. The procedure involves inserting a needle into the middle of the back and into the meninges, a membrane that contains the cerebrospinal fluid. The cerebrospinal fluid is then analyzed for evidence of infection of the brain or spinal cord. Despite sounding unpleasant and risky, a spinal tap is a well-established medical procedure and is rarely associated with any important complications. The most common complaint is a headache following the spinal tap, a complaint made less likely by lying flat once the spinal tap is completed.
T-helper cells: Synonymous with T4 cells, T4 lymphocytes, and CD4 lymphocytes. (See CD4 cells, above).
T-suppressor lymphocytes: T-suppressor lymphocytes are another class of T lymphocytes (see Immune system, above). T-suppressor lymphocytes are synonymous with T8 cells, CD8 cells, and T8 lymphocytes. All T lymphocytes participate in the body’s defenses. T8 cells primarily regulate antibody formation by the B lymphocytes. The laboratory test called the T-cell subset analysis is a count of the various types of T lymphocytes. At one time, the proportion of the number of T4 cells to the number of T8 cells was believed to show where the person was in the course of HIV infection. This proportion was commonly referred to as the
T-helperl/T-suppressor ratio. More recent work indicates that only the number of T4 cells (CD4 cells) is important, and counting T4 cells alone is usually far less expensive than doing the total subset analysis.
Thrombocytopenia: Thrombocytopenia is a low count (penia) of thrombocytes (or platelets), cells in the blood which facilitate clotting. The usual count of thrombocytes is 150,000 to 300,000 per milliliter of blood. Lower counts of 80,000 to 120,000 per milliliter are common in people with HIV infection. When the count is very low, from 5,000 to 25,000 per milliliter, bleeding problems may occur. People with HIV infection have thrombocytopenia because their bodies produce antibodies against their own platelets. Some people have no symptoms but must still be careful to avoid cuts or anything that could cause bleeding. Other people have excessive nosebleeds, excessive bleeding from cuts, and red spots the size of pinheads that come from tiny hemorrhages into the skin. Treatment is with drugs—corticosteroids (see above), AZT, gamma globulin given intravenously, alpha interferon—or with a splenectomy, the surgical removal of the spleen.
Thrush: Thrush is an infection of the mouth caused by the fungus Candida albicans. The symptoms are white patches along the gums, on the inside of the cheeks, or on the tongue. Thrush is extremely common in people with HIV infection, and is considered part of
AIDS-related complex, or ARC (see above). Thrush is easily treated with nystatin, clotrimazole, ketoconazole, or fluconazole. .
Toxoplasmosis: Toxoplasmosis is an infection caused by the parasite Toxoplasma gondii. Toxoplasma gondii is found in cat excrement and in rare meat, both of which are the most common sources of infection. About 30 percent of all adults in the United States have Toxoplasma gondii in their bodies, but the majority are unaware of it. The parasite remains dormant (see Latency, above) and rarely causes disease unless the immune system is weakened. In people with HIV infection, the most common form of toxoplasmosis is an infection of the brain called toxoplasmic encephalitis.
Tuberculosis (TB): Tuberculosis is an infection, usually in the lungs, that is far more frequent in people with HIV infection than in the general population. The bacterium that causes TB can either be dormant (inactive TB) or active (active TB). In active TB, the usual symptoms are fever and cough. People with either active or inactive TB will have skin tests that are positive for TB. In people with HIV infection, the skin test is less reliable, especially in the later stages of HIV infection when the immune system is weakened. People with HIV infection and inactive TB should receive treatment—a drug called isoniazid—to prevent active TB. People with HIV infection and active TB should receive a combination of isoniazid and rifampin.
Vaccine: A vaccine is a drug, given by mouth or by an injection, that stimulates the immune system to form antibodies to some microbe. The polio vaccine, for instance, stimulates the immune system to form antibodies against the polio virus. These newly formed antibodies now protect the person against any subsequent exposure to that microbe. Some vaccines work better than others: with the polio vaccine, protection is nearly 100 percent; with the influenza vaccine, protection is about 70 percent. Vaccines for HIV infection are being tested in people with and without HIV infection.
Varicella zoster: Varicella zoster is the virus that causes chickenpox (varicella) and herpes zoster (shingles). See Herpes zoster, above.
Virus: A virus is a tiny microbe that, unlike bacteria, can neither survive nor reproduce unless it lives in a cell. HIV is a virus that lives in CD4 lymphocytes in humans.
Wasting: Wasting is the term given—somewhat unfortunately—to the weight loss and malnutrition that often accompany HIV infection. The causes of wasting vary; they may include opportunistic infections and tumors, or they may simply be the result of HIV infection itself. In some cases, wasting is accompanied by diarrhea; the condition is then called the
diarrhea-wasting syndrome. Wasting can be an AIDS-defining diagnosis: according to the criteria of the Centers for Disease Control, an unexplained loss of at least 10 percent of the usual body weight is diagnostic of AIDS.
Western blot: The Western blot is a test for specific antibodies, in this case, for antibodies to HIV. (See above, under ELISA.)
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HIV: GLOSSARYACTG: ACTG stands for AIDS clinical trial groups (also called ACTU, with “units” instead of “groups”). ACTGs are a consortium of medical centers throughout the United States that conduct clinical trials of drugs for treating people with HIV infection. Specifically, the drugs are for treating HIV itself, for treating opportunistic infections or tumors, and for stimulating the immune system. ACTGs are funded federally through the National Institutes of Health. Taken together, the ACTGs are the largest trials of drugs for treating HIV infection in the world: they have the largest budgets, the largest number of investigators, and the largest number of participants.     AIDS: AIDS stands for acquired immune deficiency syndrome. AIDS is the late stage of an infection caused by the human immunodeficiency virus, or HIV. The virus infects the CD4 cell (also called a T4 cell, a T4 lymphocyte, and a T-helper cell), which is critical to immune defenses. As the numbers of these cells decrease, the immune system weakens until it becomes susceptible to what are called opportunistic infections and opportunistic tumors. These infections and tumors are called opportunistic because the microbes that cause them are opportunists, taking advantage of a weakened immune system. The person with AIDS is not susceptible to all infections: CD4 cells are responsible for only a certain part of the immune defenses. As a result, the person with AIDS does not get colds or flus any more frequently than anyone else; and the opportunistic infections people with AIDS do get either do not occur in healthy persons or cause illnesses that are far less severe. Opportunistic tumors are tumors that occur disproportionately frequently in persons with fewer CD4 cells.     AIDS-advocacy organizations: See Community-based organizations, below.     AIDS-defining diagnosis: A person with an AIDS-defining diagnosis has HIV infection plus an immune system weakened enough to allow one of several opportunistic infections or tumors to occur. As a result, the person is now said to have AIDS. The opportunistic infections and tumors that make up the AIDS-defining diagnoses, according to the Centers for Disease Control, include Pneumocystis pneumonia, Kaposi’s sarcoma, toxoplasmic encephalitis, cryptococcal meningitis, candidal esophagitis, infection with cytomegalovirus throughout the body, and Mycobacterium avium-intracellulare infections throughout the body. These are not the only AIDS-defining diagnoses; they are simply the most common.     AIDS dementia complex: AIDS dementia complex is the term used for the dementia that appears to result from HIV’s directly infecting the brain. Dementia means the loss of intellectual capacity, including the loss of memory, judgment, or concentration. AIDS dementia complex is relatively common in HIV infection, but usually only in the late stages.     AIDS-related complex: See ARC, below.     Amitriptyline: Amitriptyline hydrochloride (or, e.g., Elavil) is one of a group of drugs called tricyclic antidepressants that are grouped together because of their chemical similarities. Other tricyclic antidepressants include amoxapine (or Asendin), desipramine hydrochloride (or Pertofrane or Norpramin), doxepin hydrochloride (or Adapin or Sinequan), imipramine hydrochloride (or, e.g., Tofranil), and nortriptyline hydrochloride (or Aventyl Hydrochloride or Pamelor). Tricyclic antidepressants are used to treat depression and the peripheral neuropathy that causes painful feet. In many cases, the dose is arbitrary: many people start on a low dose and have the dose increased as necessary. Side effects are common, but usually not severe enough to stop treatment. The main side effects are drowsiness, weakness, and fatigue; dry mouth; constipation; and low blood pressure and dizziness. These side effects are all dose-related, meaning the higher the dose, the more severe the side effect. Because the drugs cause drowsiness, they are often given before bedtime.     Amphotericin B: The antibiotic amphotericin B is the standard treatment for many infections caused by fungi, including most of the fungi that affect people with HIV infection: Candida, Cryptococcus, Histoplasma, Coccidioides, and Aspergillus. Amphotericin B, which is given only by vein, is highly effective. Unfortunately, it is also one of the most toxic antibiotics known. The most important side effects include kidney damage, anemia (see below), disturbances in the balance of electrolytes, nausea and vomiting, fever and chills, and phlebitis or inflammation of the vein into which the drug is injected. Many of these side effects can be reduced in severity or eliminated by stopping the drug, by continuing the drug at a lower dose, or by giving other medications at the same time that will counteract the side effects. Because of amphotericin B’s toxicity, other drugs, like ketoconazole and fluconazole, are given when they are considered to be as effective or nearly as effective.     Anemia: Anemia means that the number of red cells in the blood is reduced. Red blood cells are responsible for delivering oxygen to all parts of the body. When the reduction is severe, the result is fatigue. Anemia can be caused by HIV infection itself, by an opportunistic infection, or by several of the drugs commonly taken by people with AIDS. Drugs often responsible include trimethoprim-sulfamethoxazole, other sulfa drugs, pentamidine, amphotericin B, and AZT. When the anemia is severe, it can be corrected with transfusions. When drugs are responsible, the drugs can be reduced in dose or discontinued.     Antibiotics: Antibiotics are drugs made from natural substances (as opposed to drugs made artificially) that inhibit the growth of microbes. Antibiotics may be effective against any of the classes of microbes—including bacteria, fungi, parasites, and viruses—that cause infections. Common examples of antibiotics frequently used in people with HIV infection include trimethoprim-sulfamethoxazole, other drugs containing sulfas, pentamidine, ketoconazole, amphotericin B, pyrimethamine, ganciclovir, acyclovir, penicillin, erythromycin, nystatin, clotrimazole, and AZT.     Antibody: Antibodies are proteins and are the part of the complex immune system that attacks any substance—protein or microbe—that is foreign to the body. Certain cells called B lymphocytes recognize these substances as foreign and manufacture antibodies that inactivate or eliminate the foreign substance. The foreign substance that the antibodies attack is called an antigen (see below). For most antigens, the B lymphocytes take one or two weeks to produce antibodies; for HIV, however, the time required may be months.     Antigen: Antigens are foreign material, including microbes, that the immune system responds to by manufacturing antibodies.     Aphthous ulcer: Aphthous ulcers are ulcers or sores in the mouth and occasionally in the esophagus. They are often extremely painful, they have no clear cause, and they are often cleared up by corticosteroids (see below) or other medications.     ARC: ARC stands for AIDS-related complex. ARC is a collection of conditions associated with HIV infection that do not meet the diagnostic definition of AIDS. There is no official definition of ARC. ARC is used to refer to symptoms that often, but not invariably, mean the immune system is weakening; therefore ARC often but not invariably means that the person will soon have AIDS. Common conditions of ARC include thrush, oral hairy leukoplakia, weight loss, chronic fever, chronic fatigue, and chronic diarrhea. These conditions should be accompanied by HIV infection and should have no other likely explanation. Because some of these conditions are vague and subjective, and because they may or may not mean the person will soon have AIDS, ARC is often viewed as a nebulous term.     Asymptomatic: Asymptomatic means the absence of symptoms. The asymptomatic person feels healthy.     B lymphocytes: B lymphocytes are the white blood cells—called lymphocytes—responsible for producing antibodies. B lymphocytes are distinct from T lymphocytes (including CD4 cells, also called T4 cells), which are also part of the immune system, but which work against a different group of microbes using different mechanisms.     Baclofen: Baclofen is a drug used to control muscle spasms. The most common side effect is drowsiness and, in large doses, severe sedation, lack of coordination, and lowered functioning of the heart and lungs.     Barbiturates: Barbiturates are drugs commonly used to treat insomnia, anxiety, and seizures. Examples of barbiturates are amobarbital (or Amytal), pentobarbital (or Nembutal), phenobarbital, and secobarbital. All barbiturates affect the central nervous system: low doses cause mild sedation, and high doses can lead to deep coma. When barbiturates are used for sedation, they remain effective for only about two weeks. As a result, alternative drugs are generally preferred to treat insomnia. Barbiturates’ most important role may be for controlling anxiety.     The major side effects are symptoms of central nervous system depression, including drowsiness, depression, lethargy, and hangovers. People who take barbiturates should be aware that the drug may impair their ability to perform hazardous activities. Prolonged use of high doses of the drug can cause physical dependence, psychological dependence, and tolerance (that is, higher doses of the drug are required to produce a similar effect). Discontinuing barbiturates can cause withdrawal symptoms that are similar to the withdrawal symptoms an alcoholic has when abruptly discontinuing alcohol. Other side effects include stomach pain, allergic reactions, and fever.     Benzodiazepines: Benzodiazepines are a class of drugs commonly used to treat anxiety, insomnia, seizures, and painful muscles. Examples of benzodiazepines include alprazolam (or Xanax), diazepam, flurazepam hydrochloride (or Dalmane), lorazepam (or Ativan), midazolam maleate, oxazepam (or Serax), prazepam (or Centrax), temazepam (or Restoril), and triazolam (or Halcion). In general, all benzodiazepines act in similar ways and seem to be equally effective. Most physicians prefer benzodiazepines for treating anxiety and tension. Compared to barbiturates and meprobamate, and when given at the doses that relieve anxiety, they are less addictive and produce less sedation. The major side effects are drowsiness, loss of coordination, confusion, dizziness, and fainting. People taking benzodiazepines should be aware that the drug may impair their ability to perform activities that require mental alertness and physical coordination. Benzodiazepines can also cause physical dependence and symptoms of severe withdrawal if the drug is stopped suddenly after being used regularly for a long time.     Biopsy: Biopsy is a procedure for obtaining a piece of tissue for examination under the microscope. The microscopic changes in tissue often provide a diagnosis, and stains and cultures for microbes will often reveal the infecting organism. The biopsy may be obtained using lidocaine to deaden the skin to avoid pain. The biopsy may be performed on an outpatient basis when the area to be biopsied is near the surface or when it is in the lungs or gastrointestinal tract and can be reached through an endoscope, an instrument passed through the mouth or anus. Alternatively, the biopsy of organs deep within the body may require an operating room procedure.     Bleach: Chlorine bleach is highly effective in killing HIV within minutes. It is available at most grocery stores and is commonly recommended for killing any virus or other microbe that may be in such body fluids as blood, saliva, and stool. Bleach is usually diluted 1:10, or one part of bleach in ten parts of water. This dilution can be applied to surfaces or in the washing machine for clothes.     Blood count: Blood is composed of red blood cells (erythrocytes) which carry oxygen to all parts of the body, white blood cells (leukocytes) which help make up the immune system, and platelets (or thrombocytes) which are required for blood clotting. All three kinds of cells can be counted under a microscope. A low red blood cell count is called anemia (see above); a low white blood cell count is called leukopenia (see below), and a low platelet count is called thrombocytopenia (see below). People with HIV infection commonly have low red counts, low white counts, and low platelet counts. A blood count is a routine procedure for clinical laboratories; it is a relatively simple, inexpensive, and standard test to evaluate people with HIV infection.     Bone marrow: Bone marrow is the tissue in the central portion of many bones where blood is manufactured. Bone marrow can be withdrawn (by placing a needle in the hip bone) and analyzed to detect abnormalities in the production of red blood cells, white blood cells, or platelets.     Buyers’ clubs: Buyers’ clubs are groups, or even individuals, that make underground drugs (drugs not approved by the Food and Drug Administration and thus not available as licensed drugs in the United States) available to people with HIV infection. The advantage of a buyers’ club is that it offers the person with HIV infection the opportunity to take drugs that might require long periods of testing before becoming widely available. The disadvantage is that the drugs are untested and could be toxic or useless.     Candidiasis: Candidiasis is an infection caused by the fungus Candida albicans. People with HIV infection commonly have candidiasis in the mouth (thrush), in the esophagus (candidal esophagitis), or in the vagina (vaginal candidiasis). The diagnosis can be confirmed by microscopic examination of the patches. Candidiasis is common in people who do not have HIV infection. In people who do have HIV infection, candidiasis is especially common, sometimes severe, and likely to recur. Treatment with nystatin, clotrimazole, or ketoconazole—all taken by mouth—is usually successful. When the infection is extensive or when it involves deeper organs, it is sometimes necessary to treat with intravenous amphotericin B.     CD4 cells: The blood contains several kinds of white cells, each of which plays a specific role in the immune system. CD4 cells (other names are T4 cells and T-helper cells) are the cells that HIV selectively infects. The number of CD4 cells frequently indicates the stage of HIV infection. Healthy people without HIV infection usually have around 1,000 CD4 cells in every milliliter of blood; counts of 400-700 are considered abnormally low, but not alarming. People with AIDS usually have counts of less than 200. Counts of less than 200 do not indicate AIDS, but suggest severe weakening of the immune system. The CD4 count is a relatively expensive test, but it is an important way of monitoring the state of the immune system. In any one person, however, the count varies considerably: the same laboratory performing the test on the same specimen can show counts that vary by as much as 20 percent. The CD4 count is also influenced by a variety of other medical conditions independent of HIV infection. As a result, although the CD4 count is frequently used to assess progressive disease, changes in the count are sometimes difficult to interpret.     Centers for Disease Control {CDC): The Centers for Disease Control is a federally funded institution located in Atlanta, Georgia. It has three responsibilities: to serve as an epidemiologic and public health resource for state and local health departments; to investigate epidemics; and to keep track of contagious diseases and other diseases important to public health. The CDC has about 4,000 employees, including 800 physicians and Ph.D.’s. In the past, the CDC has been responsible for much of what we know about Lyme disease, Legionnaires’ disease, and toxic shock syndrome. More to the point, the CDC provided much of the early epidemiologic data that identified the symptoms of AIDS, the kinds of behavior that risked AIDS, and how AIDS was transmitted—in fact, the CDC was responsible for the name AIDS. At present, the CDC is the storehouse for all reported cases of AIDS in the United States. It provides guidelines for disease prevention and gives advice on safety for health care providers. It is responsible for funding state and local agencies that test for HIV, counsel, and collect data.     Chloral hydrate: Chloral hydrate is a sedative used to treat insomnia. It is usually taken fifteen to thirteen minutes before bedtime. Using chloral hydrate regularly for more than two weeks often reduces its effectiveness. Major side effects include stomach irritation, residual sedation, or a hangover. Chloral hydrate should be used with great caution in people who are depressed, who may commit suicide, or who have a history of drug abuse.     Clostridium difficile: People who take antibiotics often develop diarrhea as a side effect. A relatively common and particularly severe cause of this diarrhea is a microbe called Clostridium difficile. Almost any antibiotic can cause this complication, but the most frequent causes are ampicillin, amoxicillin, clindamycin, and a group of drugs called cephalosporins that includes cefixime (or Suprex), cefuroxime, cephalexin (or Keflex), and cefaclor (or Ceclor). People who have diarrhea while taking these or any other antibiotics should stop taking the antibiotics and call their physicians. A test of stool will determine if Clostridium difficile is the cause. If it is, it can be treated with metronidazole or vancomycin hydrochloride. Vancomycin is preferred for serious cases of diarrhea, but it costs about $200 to $300. Metronidazole is less expensive—$10 to $20—and equally effective unless the person has severe colitis.     CMV: CMV, which is short for cytomegalovirus, is a virus commonly found in people without HIV infection. Usually the immune system holds CMV in check, and it remains dormant in the body without causing any serious disease. With a severely weakened immune system, however, CMV may cause serious infection. The site of the infection can be in the eye, lung, liver, gastrointestinal tract, bone marrow, brain, or widespread in many of these areas. The virus can be detected by cultures of blood, cultures of urine, or biopsies of any of the organs that are affected. CMV is difficult to treat: some antibiotics seem to inhibit its growth in the test tube, but no one agrees about when these drugs should be used. An exception is CMV of the retina in the eye: CMV retinitis may result in loss of vision and requires treatment with a drug which slows the progression of the infection and which must be taken indefinitely.     Co-factor: A co-factor is anything—microbes, proteins, hormones, genes—which makes a disease progress more rapidly. With HIV infection, co-factors are only suspected but may include other viruses (like cytomegalovirus), age, genetic resistance or predisposition, and certain hormone-like substances, called cytokines, released by lymphocytes.     Colon: The gastrointestinal tract—which starts at the mouth and ends at the rectum—includes the esophagus, stomach, small intestine, colon, and rectum. The colon and the small intestine are commonly the sites of infections that cause diarrhea. To diagnose problems in the colon, common procedures are colonoscopy and sigmoidoscopy. These procedures permit visualization and biopsy of the colon by passing a tube through the rectum.     Community-based organizations (CBOs): Community-based organizations are also called AIDS-advocacy organizations and AIDS service organizations (ASOs). They are organizations and agencies that provide services to people with HIV infection, as well as education and prevention programs for the whole community. The leaders of community-based organizations are lay people, ordinary people who do not come from the government or from organized medicine—although many community-based organizations have physicians as advisers, and most receive public funds.     Examples of community-based organizations dealing with other diseases are the American Lung Association, the American Heart Association, and the American Cancer Society. There is no similar nationwide organization for people with HIV infection or AIDS. Nevertheless, most cities have one or sometimes several such organizations: examples include Shanti in San Francisco, the Gay Men’s Health Clinic in New York City, and HERO in Baltimore. The types of services offered vary but may include counseling, crisis support, financial assistance, transportation, meals, housing, support groups, legal aid, social services, education, psychological support, hotlines, buyers’ clubs, and medical services. Most of these organizations have a paid professional staff but rely heavily on volunteers. Funding usually comes from state governments, corporations, foundations, and local fundraising events.     Computerized tomography scan (CATscan): CAT scans are a particular kind of x-ray that provide a three-dimensional view of the body. Conventional x-ray tests provide a two-dimensional view of the body; CAT scans use computers to stack a series of two-dimensional x-rays together to form a three-dimensional image of the body. CAT scans can be done of the entire body or of parts of it. The person receiving a CAT scan first receives an injection of what is called contrast material—material that shows up under x-rays. Some people have allergic reactions to contrast materials and should not receive them again. The person receiving the CAT scan is next put into a chamber with a scanner that circulates around the body, producing three-dimensional images in parallel sections of about an inch or less. CAT scans, first developed in the 1970s, are an excellent method for detecting tumors, infections, or other changes in the anatomy of the brain, chest, abdomen, or other parts of the body. They are also expensive, usually costing around $300 to $800.     Constitutional symptoms: Symptoms caused by the impact of an illness on the entire body or constitution are frequently referred to as constitutional symptoms. Included are fatigue, achiness, weight loss, fever, and night sweats. Constitutional symptoms are present in many types of infectious diseases, tumors, and other medical conditions ranging from the serious to the trivial. For people with HIV infection, constitutional symptoms may be a result of HIV infection itself or the result of such opportunistic illnesses as Pneumocystis pneumonia, tuberculosis, or widespread CMV infection.     Contagious: A disease that is contagious can be passed from one person to another. A disease that is infectious is caused by a microbe. All diseases that are contagious are also infectious; but some diseases, like toxic shock syndrome, are infectious and not contagious. HIV is both infectious and contagious, but is contagious only with specific types of contact.     Corticosteroids (also known as steroids, glucocorticosteroids, prednisone, and cortisone): Corticosteroids are drugs used to reduce the immune response. Numerous preparations are available that can be taken intravenously, by mouth, or in an ointment applied to the skin. Using high doses of corticosteroids for a long time can be dangerous: they reduce the immune system’s defenses against certain infections. Corticosteroids are sometimes considered especially dangerous for people with HIV infection, whose immune defenses are already weakened. Nevertheless, many of the complications of HIV infection appear to result from an overly abundant but misdirected immune response. As a result, these complications of HIV infection respond well to corticosteroids, though the drug should be taken at the lowest doses for the shortest period.     Cryptococcosis: Cryptococcosis is an infection caused by the fungus Cryptococcus neoformans. This fungus can cause infection in otherwise healthy people. In people with HIV infection, however, it is especially severe, frequently causing meningitis. Common symptoms include headache, fevers, vision problems, and seizures. The diagnosis is usually made by analyzing cerebrospinal fluid obtained with a spinal tap (see below). The disease is treated with amphotericin B given by vein or fluconazole given by mouth; when treatment is stopped, the disease tends to recur so that long-term treatment is generally necessary.     Cryptosporidiosis: Cryptosporidia are parasites that infect the intestine and cause diarrhea. This infection, called cryptosporidiosis, can occur in otherwise healthy persons, but the diarrhea generally does not last long and is not severe. Cryptosporidiosis in people with HIV infection often causes devastating diarrhea that persists for months. People with cryptosporidiosis may lose large amounts of fluid and nutrients and, consequently, become severely malnourished. The diagnosis is usually established by simply examining the stool under a microscope to detect the parasite. There is no universally accepted form of treatment except to replace the lost fluids and nutrients.     Culture: A culture, in medical terms, is a medium in which microbes can grow. HIV is grown in cultures containing lymphocytes. If a sample of a person’s blood is put into such a culture, and HIV grows, the person is infected with HIV. Other blood tests for HIV are polymerase chain reaction (see below) or the P24 antigen test (see below). The usual blood test for HIV detects antibodies to the virus instead of the virus itself. The antibody test is usually preferred because it is less expensive, better standardized, and more readily available.     Cytomegalovirus: See CMV, above.     Dantrolene (or Dantrium): Dantrolene is one of several muscle relaxants. The most common side effect is muscle weakness that usually disappears after taking the drug for several days. Other side effects include hepatitis (see below), diarrhea, gastric intolerance, depression, insomnia, and frequent urination.     Dementia: See AIDS dementia complex, above.     Dextroamphetamine sulfate (or Dexedrine): Dextroamphetamine sulfate, along with methylphenidate hydrochloride (or Ritalin Hydrochloride), stimulates the brain. It is usually given to people with HIV infection with AIDS dementia complex to counter the symptoms of apathy and social withdrawal. The most common side effects are nervousness and insomnia. Both side effects can usually be controlled by decreasing the dose and by avoiding taking the drug late in the day.     Dormant: See Latency, below.     Dysphagia: Dysphagia means difficulty with swallowing. The most common cause of dysphagia is an infection by Candida albicans, a fungus that can be easily treated (see Candidiasis, above). Less frequent causes are infections with herpes or CMV. In some people dysphagia has no readily apparent cause. The usual method of finding the cause of dysphagia is endoscopy, a procedure in which a tube is placed in the esophagus to visualize and biopsy the lesions. X-ray examinations are another means of viewing the esophagus. In many cases, neither of these tests is done, and the person is presumed to have a Candida infection if he or she also has thrush (see below) and if swallowing is painful.     ELISA test: The ELISA (pronounced eelissa) is a blood test done to detect antibodies to certain microbes, among which is HIV. The ELISA is the first of two standard tests done together to detect antibodies to HIV. The test is extremely sensitive but not very specific. Sensitivity means that the test is able to detect HIV infection; specificity means that the test specifically detects a particular infection and no other. In other words, with ELISA, people who have HIV infection will rarely have a falsely negative test, but people who do not have HIV infection will commonly have a falsely positive test. As a result, the ELISA is used as a screening test, and those who are positive have a second test on the same blood sample called a Western blot.     The Western blot test, combined with an ELISA, is over 99 percent accurate in both sensitivity and specificity. The combination of tests is generally offered free of charge from most health departments and at a cost of $20 to $150 from commercial laboratories. The test offered may be anonymous, meaning that the person receiving the test cannot be identified, or it is confidential, meaning that privacy is honored but a record is kept identifying a specific person with the test result. The ELISA is easily performed, but the Western blot is more complicated and often done only by reference laboratories or on certain days of the week. For this reason, the results may not be available for several days or even weeks. The test results are usually either positive or negative, but occasionally people have Western blots that can not be clearly interpreted and the test results are considered indeterminant. The usual recommendation for people with indeterminant results is to have the test repeated in two or three months. People at a low risk for HIV and with indeterminant results almost never have HIV infection, and the cause of the indeterminant results is not known.     Encephalitis: Encephalitis is an infection of the brain. (Meningitis, by contrast, is an infection of the meninges, the membrane surrounding the brain and spinal cord—see Meningitis, below.) Encephalitis commonly causes headaches, fever, seizures, and neurologic problems. The diagnosis is frequently made on the basis of the person’s symptoms, combined with procedures to examine the brain such as computerized tomography scan [CAT scan) (see above); magnetic resonance imaging (MRI) (see below); or electroencephalogram (EEG). Diagnosis can also be made by analyzing the cerebrospinal fluid obtained by a spinal tap (see below). In people with HIV infection, the usual causes of encephalitis are infection with HIV itself or such opportunistic illnesses as toxoplasmosis or lymphoma.     Endoscopy: Endoscopy is a diagnostic procedure in which an instrument is passed through the mouth or rectum to examine an internal organ or to obtain a biopsy (see above). In people with HIV infection, the most common types of endoscopy are bronchoscopy to examine the lungs and endoscopies to examine the digestive system. Upper endoscopy of the intestine involves passing an endoscope through the mouth to examine the esophagus, stomach, or upper small intestine. Lower endoscopy of the intestine involves passing an endoscope through the rectum to examine the large intestine or colon. Endoscopes are flexible and can turn corners. Endoscopy requires the expertise of a specialist, can be done on an outpatient basis, and usually costs $1,200 to $1,800.     Enteritis: Enteritis is an inflammation of the small intestine; the most common symptom is diarrhea. In people with HIV infection, the microbes that usually cause enteritis are Cryptosporidia, microsporidia, Mycobacterium avium-intracellulare, and CMV. These microbes can be detected by examining stools under a microscope or with a biopsy of the small intestine done with an endoscope (see above, under Endoscopy), a tube that is placed through the mouth and into the small intestine.     Epidemic: An epidemic is a disease that occurs in many more people than would be expected during a given time. Epidemiology is the study of the factors that determine the frequency and distribution of diseases.     Fluconazole (or Diflucan): Fluconazole is used to treat fungal infections, primarily those caused by Candida albicans (thrush or candidal esophagitis) and Cryptococcus neoformans (cryptococcal meningitis). Fluconazole can be taken by mouth or by vein. Side effects are unusual; occasional problems are nausea, rash, or hepatitis.     Ganciclovir: Ganciclovir is used to treat infections caused by cytomegalovirus and occasionally for infections caused by herpes simplex and other viruses. It is given only intravenously. The most important side effect is a low blood count, especially neutropenia, which predisposes the person to bacterial infections (see Blood count, above, and neutropenia, below, under Leukopenia). If neutropenia is severe enough, the dose of the drug should be reduced, or the drug should be temporarily stopped.     Hemophilia: A person with hemophilia lacks a protein that helps the blood to clot. Hemophiliacs bleed easily, even with a trivial cut; many have severe hemorrhaging into the joints and eventually get joint disease. Hemophilia is inherited, and only by men; the gene for hemophilia is carried by women, who do not get the disease but who can pass the gene on to their sons.     Hemophilia has two forms, hemophilia A and hemophilia B; each form lacks a different clotting protein, called a clotting factor. Hemophilia is treated by substituting a commercial clotting factor for the clotting factor the blood lacks. The commercial clotting factor is extracted chemically from blood donated by hundreds or thousands of people. As a result, hemophiliacs are exposed to the blood of thousands of donors. Between 1978 and 1985, from the time HIV was introduced into the United States until the time the blood banks screened for HIV, hemophiliacs had a high risk of being infected with HIV. Approximately 70 percent of men with hemophilia A and 30 percent of men with hemophilia B acquired HIV infection from infected commercial clotting factors.     Since 1985, the risk of being exposed to HIV through clotting factors has dropped to practically nil. One reason is that donated blood is now screened for HIV; another reason is that clotting factors are heated and purified by detergents and biochemicals which kill HIV. The Centers for Disease Control found that between 1985 and 1988, only 18 hemophiliacs acquired    HIV, an annual rate of under one per thousand.     Hepatitis: Hepatitis is an inflammation of the liver. Many people have no symptoms and are unaware of having hepatitis. The symptoms, when people do have them, are loss of appetite, vomiting, yellow discoloration of the skin and eyes (jaundice), dark urine, sore stomach, and fever. Hepatitis is usually caused by a virus called hepatitis B virus (see below) that may be transmitted by sexual contact or blood-to-blood transmission. Since these are the same mechanisms of transmitting HIV infection, the same people who are likely to be infected with HIV infection are also likely to be infected with the hepatitis B virus. People with HIV infection are also prone to hepatitis caused by CMV and Mycobacterium avium-intracellulare. Alcohol and drugs, including AZT, pentamidine, ketoconazole, trimethoprim-sulfamethoxazole, and INH, may also cause liver inflammation. The diagnosis of hepatitis is easily made with blood tests to determine liver function and to detect specific microbes, including hepatitis B, hepatitis C, and hepatitis A viruses. When the cause is unclear, it is sometimes helpful to obtain a biopsy of the liver or to do tests of the gall bladder.     Hepatitis B virus: The hepatitis B virus is one of the microbes that causes hepatitis. Hepatitis B infection may be acute and cause serious symptoms that last up to a few weeks (see above, under Hepatitis); it may be chronic with occasional symptoms and abnormal liver tests that last for months or years; or it may cause no symptoms at all and may only show up on a blood test. About 5-10 percent of people with hepatitis B infection become chronic carriers of hepatitis B virus; they continue to carry the virus and can transmit it to others for years. The hepatitis B virus is transmitted the same way HIV is, by sexual contact or blood-to-blood transmission.      Hepatitis B is transmitted far more efficiently than HIV, so that a person exposed by a needlestick accident involving a person with both infections is about twenty times more likely to develop infection with the hepatitis virus. The blood supply used for transfusions is screened for the hepatitis B virus and is therefore an unlikely source of this infection.     There is no evidence that hepatitis B is any different in people with HIV infection than in people without HIV infection. However, the presence of liver damage or ongoing inflammation may complicate the use of certain drugs that (like AZT) require the liver for metabolism or that (like AZT and pentamidine) may occasionally cause further liver damage (see Hepatitis, above). Once infection takes place, no treatment will eradicate the hepatitis virus. Infection may be prevented, however, by a vaccine. The vaccine is recommended for the people at risk for this infection: people who share needles to inject drugs, people who practice unsafe sex with gay men, family members who live in the same household, sex partners of people known to be hepatitis B carriers, and health care workers. Three injections are required, at a cost of about $100 for all three doses.     Herpes simplex virus: Herpes simplex is a virus that commonly causes infections of the mouth and genitals. There are actually two different viruses: though similar in many respects, one kind seems mostly likely to infect the mouth and the other, the genitals. The symptoms of both infections are blisters on the mouth or genital area that first contain clear fluid, then become filled with pus, finally form scabs, and eventually disappear. Herpes simplex is a persistent virus: the virus remains dormant most of the time and then causes recurrent symptoms intermittently over a period of years. The initial infection with herpes simplex virus is often severe with large areas of blisters, occasional fevers, and pain and tingling in the area involved. Subsequent attacks are usually milder. The virus is transmitted to others by contact with the mouth or genitals, especially when the blisters are present.     Both the oral and the genital form of herpes are common infections in the general population; in people with HIV infection, however, the blisters are likely to be more severe, be spread over relatively larger areas, and, most importantly, persist for longer periods of time. In a person with HIV infection, herpes blisters that persist over one month constitute an AIDS-defining diagnosis. Treatment with a drug called acyclovir usually heals the blisters, prevents recurrences, and reduces the risk of transmitting the virus to others. Acyclovir is available as an ointment to put on the blisters and as tablets to be taken by mouth. In people with AIDS who have severe herpes infections, acyclovir is also given intravenously; once the infection is under control, the tablets are often given for extended periods to prevent recurrences.     Herpes zoster: Herpes zoster is caused by the same virus that causes chickenpox. The virus persists in the body and may cause symptoms decades after the original infection. Attacks after the first infection are called shingles, or herpes zoster. The skin sores with herpes zoster are similar to those of chickenpox and those of herpes simplex. The sores begin as red spots that become blisters filled with water; the blisters break down into sores with pus, finally scab over, and eventually disappear. Unlike herpes simplex infections or chicken-pox, however, the later recurrences of herpes zoster are usually restricted to the area served by a single nerve. In other words, the blisters are restricted to one side of the body, usually in a band across the face, chest, abdomen, back, or leg.     In many people, recurrences of herpes zoster are accompanied by post-herpetic neuralgia, a pain at the site of blisters that may persist for months after the blisters are gone. Post-herpetic neuralgia is fortunately infrequent among people with HIV infection. Herpes zoster is more common and more severe in people with HIV infection. It does not, however, necessarily mean that the immune system is weakening, and it clearly does not indicate AIDS. The diagnosis is generally made with a microscopic examination and culture of blisters, but the appearance of the blisters is usually all a physician needs to make a diagnosis. Acyclovir appears to hasten healing, but high doses of the drug must be given by mouth or by vein.     Hickman catheter: People who require long courses of drugs given by vein will often have a tubing called a Hickman catheter. The catheter is inserted by a specialist, usually a surgeon, through the skin of the chest, and then tunneled under the skin to a vein in the chest. The end of the catheter comes out the chest wall above the breast. Drugs can be injected into the catheter as necessary. The advantage of a Hickman catheter is that it permits access to the vein without repeated needlesticks in the arms. Similar devices are also available; the Hickman catheter is simply one of the most frequently used.     It is important to know that the area around any catheter in a vein can become infected. Symptoms of infection of the area where the catheter comes out are redness and pain, and sometimes pus. Symptoms of infection around the catheter inside the body are feverand chills. Anyone with a Hickman catheter and these symptoms should tell a physician right away. Antibiotics should be given immediately, and sometimes the catheter needs to be removed.     HIV: HIV stands for the human immunodeficiency virus. HIV has had several names. It was first called lymphadenopathv associated virus (LAV) by Luc Montanier in France in 1983, and next called human T-lymphotropic virus III (HTLV-III) by Robert Gallo in the United States in 1984. HIV is now the official international name of this virus. HIV is the virus responsible for AIDS. There are occasional arguments that perhaps HIV does not cause AIDS or is responsible for AIDS only in combination with other viruses, but at present, the great majority of scientific authorities accept HIV as the sole cause of AIDS. The only other causes are the closely related viruses such as HIV-II that are responsible for a very few cases of AIDS, primarily in Africa.     Idiopathic thrombocytopenic purpura (ITP): See both Platelets and Thrombocytopenia, below.     Immune system: The human body is defended against a multitude of microbes by a complex system called the immune system. The principal components of the immune system are cells called B lymphocytes, neutrophils, and T lymphocytes. B lymphocytes make antibodies, the proteins that attack bacteria and viruses; neutrophils envelop and kill bacteria; and T lymphocytes provide communication between the parts of the immune system. Although these three components are somewhat interdependent, each takes primary responsibility for defense against certain types of microbes. For this reason, people deficient in different components are prone to infections with quite different microbes.     The cell type that is primarily affected in people with HIV infection is a type of T lymphocyte called a CD4 cell (see above). The most common infections encountered in people with few CD4 cells are caused by Pneumocystis carinii, cytomegalovirus, Mycobacterium avium-intracellulare, herpes simplex virus, herpes zoster, Candida albicans, Toxoplasma gondii, Cryptosporidium, Cryptococcus, Salmonella, and the bacterium that causes tuberculosis. People with immune systems weakened by HIV are not only subject to high rates of infections with these organisms, but the infections tend to be severe, prolonged, and recurrent. At the same time, many other microbes that commonly cause infections in everyone do not appear to be unusually common or severe in people with HIV, presumably because the other components of the immune defenses remain relatively strong.     Incubation period: The incubation period of a disease is the time interval between infection with a microbe and the first symptoms of disease. For influenza and common colds, the incubation period is usually several days; for measles, chickenpox, mumps, and infections caused by many other viruses, the incubation period is two to three weeks. An unusual feature of HIV infection is that the first symptoms of a weakened immune system usually do not occur until several years after the infection takes place.     Infectious: See Contagious, above.     Influenza vaccine: The influenza vaccine varies in effectiveness, depending on whether the strain of virus in the vaccine is related to the virus which is causing the influenza. The effectiveness of the vaccine changes every year. In most years, however, the vaccine probably prevents about 70 percent of the cases of influenza, and those who become infected despite having been vaccinated usually have less severe symptoms. Influenza does not seem to be unusually common or severe in people with HIV infection. The only problem specific to people with HIV infection is that the symptoms of influenza can be confused with the symptoms of other pneumonias such as pneumocystis pneumonia (see Pneumocystis carinii, below), a confusion it would be nice to avoid. Therefore, the CDC’s Advisory Committee on Immunization Practices recommends that people with HIV infection routinely get the influenza vaccine every year.     Informed consent: Informed consent is a form of protection for people considering taking an HIV antibody test or undergoing certain medical procedures (like an operation) or considering participation in a clinical trial. Before taking the test, undergoing the procedure, or participating in the trial, the person or the person’s representative must sign an informed consent form stating that Tie or she has been informed about the purpose, benefits, risks, and alternatives to the test, procedure, or trial, and that he or she consents to it. In the case of participation in a clinical trial, the informed consent form explains the purpose of the trial, what will be done, the risks of participation, the benefits of participation, what other treatments are available, and the right of the participant to leave the trial at any time.     Inoculum size: Inoculum size is a term used in the field of infectious diseases to describe the number of microbes necessary to cause an infection. In HIV infection, for example, a certain number of viruses is required before infection takes place. The specific number is not known. What is known is that the probability of transmitting HIV with the transfusion of one unit (or 500 milliliters) of infected blood is 80 to 90 percent. The probability of transmitting HIV with a needlestick injury, which injects only a fraction of a milliliter of blood, is 0.4 percent. This difference in the probabilities of transmission is most likely due to differences in inoculum size.     Interferons: Interferons are proteins that cause cells to resist attack by certain viruses. Interferons are usually produced by the body, but they are also made artificially and used as medications. The major side effects of interferons are the achiness and fever that accompany flu: it is the interferon produced by the body that causes these symptoms during flu.     Isoniazid (INH): Isoniazid is the standard drug used to treat and prevent tuberculosis. Isoniazid is usually recommended for any person with HIV infection who has tuberculosis or who has a positive tuberculosis skin test. The usual dose is 300 milligrams, taken by mouth. The most important side effect is hepatitis, including jaundice (yellowish skin and eyes), dark urine, nausea, and abdominal pain. This side effect is more likely in people who already have liver damage for other reasons, and in older people. People taking isoniazid and having these symptoms should stop taking the drug immediately and call their physician.     Kaposi’s sarcoma: Kaposi’s (pronounced kaposhee’s) sarcoma is a tumor of blood vessels. Next to Pneumocystis pneumonia, it is most likely to be the first AIDS-defining diagnosis that people have; approximately 20 percent of all people with AIDS have Kaposi’s sarcoma. The symptoms of Kaposi’s sarcoma are purplish nodules, a quarter of an inch to an inch in diameter, anywhere on the skin. The nodules will grow in size and number. They sometimes occur on internal organs like the lung, brain, and gastrointestinal tract, though they often cause no specific symptoms at these sites. Some nodules are painful. The face and legs may swell if the lymph channels nearby are blocked. If Kaposi’s sarcoma becomes extensive, people may have fever, weight loss, and severe fatigue.     The diagnosis can be established by a biopsy of the nodules. Biopsies are easy to do with nodules on the skin, but more difficult when the nodules are on internal organs. The main reason to do the biopsy is that the nodules might possibly turn out to be something other than Kaposi’s sarcoma; and if they are Kaposi’s sarcoma, they are an AIDS-defining diagnosis. Therapy is controversial: Kaposi’s sarcoma is rarely life-threatening, and treatment is neither easy nor universally effective. If the nodules are painful, disfiguring, or complicated by swelling, they can be treated with radiation or interferon; if the nodules have spread widely over the skin or into internal organs, causing symptoms, they can be treated with the same drugs given to people with cancer.     Ketoconazole: Ketoconazole is a drug given by mouth for infections caused by Candida albicans and other fungi. Ketoconazole requires acid in the stomach to be absorbed into the system. People should therefore not take other medicines that neutralize stomach acids until at least two hours after the dose of ketoconazole. Side effects include nausea, vomiting, hormonal problems (menstrual problems and reduced sex drive), rash, headaches, and liver damage.          Latency: Latency and dormancy (which literally means sleeping) mean the same thing: a microbe is in the body but is not actively reproducing, not invading any tissues, and not causing symptoms. Examples of microbes that are latent or dormant in many or most healthy people are: Pneumocystis carinii, Toxoplasma gondii, herpes simplex virus, the virus that causes herpes zoster, and cytomegalovirus. Once in the body, these microbes remain in the body. They remain latent or dormant until something tilts the balance in the immune system and permits them to become active.     Leukopenia: Leukopenia means a low number (or penia) of white blood cells (or leukocytes—leuko means white), the cells of the immune system that fight infection. Leukocytes include lymphocytes (cells that recognize foreign material) and neutrophils (cells that gobble up microbes). The normal leukocyte count is 4,000 to 8,000 per milliliter of blood. In people with certain infections, especially with bacterial infections, the leukocyte count is high (leukocytosis). In people with viral infections, including HIV infection, the leukocyte count is low (leukopenia). Having a low count of lymphocytes is called lymphopenia; lymphopenia is the expected result of HIV infection. A low count of neutrophils is called neutropenia; neutropenia can be caused by HIV itself or by some of the drugs commonly taken during HIV infection. Neutropenia becomes worrisome if the count is less than 750 per milliliter; if the count is less than 500 per milliliter, the person is prone to bacterial infections.     Lumbar puncture: See Spinal tap, below.     Lymph glands: The lymphatic system is a widespread network, like the blood circulation system, of channels that carry lymph. Lymph is a clear fluid containing lymphocytes, or white blood cells (including CD4 cells), that are a part of the immune system. Lymph is manufactured in the lymph glands, which are clumps of lymphatic tissue distributed widely throughout the body. When lymph glands are near the surface of the skin, they can be felt as bumps below the skin’s surface. The usual locations where they can be felt are the back of the neck, below the jaw, under the armpits, and in the groin. Lymph glands are commonly swollen and sometimes painful and tender when they are infected. Many infections involve the lymph nodes. In HIV infection, swollen lymph glands are likely to occur in three different circumstances: with persistent generalized lymph adenopathy, or PGL (see below), in which many lymph glands are swollen for months; with infection of the lymph glands by certain opportunistic diseases; and with lymphomas, which are tumors of the lymphatic system seen more frequently in people with HIV infection than in the general population. Swollen lymph glands may require diagnostic tests: the usual is a biopsy of the lymph gland or removal of the whole gland to permit microscopic examination of the lymphatic tissue.     Lymph adenopathy: Lymphadenopathy means swollen lymph glands. Swollen lymph glands are most common at the back of the neck, along the jaw, in the armpits, and in the groin. The lymph glands may feel like rubbery, discrete nodules that are rarely tender to touch and often pea-sized; glands of this description are common in everyone and in several conditions unrelated to HIV infection. If they are swollen to abnormal size for longer than a month in at least two different areas, they constitute persistent generalized lymphadenopathy (PGL) (see below).     Lymphoma: Lymphoma is a cancer of the lymphatic system. Lymphoma occurs most frequently in people without HIV infection, but people with weakened immune systems, including those with HIV infection, have lymphomas about forty times more frequently than normal. About 1 to 3 percent of people with AIDS have lymphomas, and for people with AIDS, lymphomas are classified as opportunistic tumors. There are many types of lymphomas: some progress extremely slowly, cause few symptoms, and require minimal treatment; some are more severe. People with AIDS generally have lymphomas called non-Hodgkin’s lymphomas of B cell origin. These lymphomas tend to be severe, and they also tend to involve unusual areas of the body like the brain, liver, kidneys, intestines, and lungs. The diagnosis is usually established with a biopsy. Treatment is variable and often requires the assistance of a specialist in cancer treatment using cancer chemotherapy or radiation treatment.     Magnetic resonance imaging (MRI): Magnetic resonance imaging is a technique used to make a three-dimensional image of the interior of the body. Though the technique is somewhat different from a CAT scan (see Computerized tomography scan, above), the images are similar. The person getting an MRI is placed inside a large tubular structure and remains motionless for thirty to sixty minutes: the worst problems are boredom and claustrophobia. During that time, the person’s body is bathed in a magnetic field, which causes the atoms in different tissues to give off tiny radio signals. The signals are different depending on the kind of tissue. An MRI is better than a CAT scan at detecting diseases of the brain and spinal cord. MRI is painless, harmless, and does not involve exposure to radiation; the body is not exposed to any kind of potentially harmful radiation. MRIs are also expensive, from $500 to $1,000.     Megestrol Acetate (Megace): Megestrol is a drug that stimulates the appetite. The drug has virtually no serious side effects, even with doses as high as 800 milligrams daily.     Meningitis: Meningitis is an infection of the meninges, the membrane that envelopes the brain and spinal cord. The most common cause of meningitis in people with HIV infection is Cryptococcus (see under Cryptococcosis, above).     Meprobatnate (e.g, Equanil, Miltown): Meprobamate is a drug that acts on the central nervous system much as barbiturates do. Meprobamate is used most commonly to treat anxiety. As with barbiturates, meprobamate can cause drowsiness, lethargy, and lack of coordination. Continued use for weeks or months may cause tolerance—that is, increasing doses are required for the same effect. Continued use may also cause psychological and physical dependence. Withdrawing the drug suddenly after prolonged, regular use may cause severe reactions. Other side effects can include stomach irritation and allergic reactions.     Methadone hydrochloride: Methadone is an opiate that is commonly used to control narcotic withdrawal symptoms and to maintain people addicted to morphine-like drugs, particularly heroin. Methadone maintenance is permitted only in programs approved by the Food and Drug Administration and the designated state authority.Methadone can be given by mouth or by vein. Side effects are those shared by all morphine-like drugs that depress the central nervous system: dizziness, mental clouding, depression, and sedation. Methadone may cause physical dependence. If it is stopped abruptly after prolonged and regular use, it can cause withdrawal symptoms.     Metronidazole (e.g., Flagyl): Metronidazole is an antibiotic taken by people with HIV infection for common intestinal infections and common dental problems like gingivitis (inflammation of the gums) and periodontitis (infection of the structures that support the teeth). The drug is given by mouth or by vein. Side effects are unusual, primarily nausea and stomach pain. The side effects can improve if the drug is taken with meals or if the dose is reduced. Taking this drug for periods of months may cause pain in the feet that resembles the pain of HIV neuropathy (see below). This pain usually goes away when the drug is stopped.     Microbes: Microbes are organisms so small they require a microscope to be seen. They can be bacteria, viruses, parasites, or fungi. HIV is one example of a virus. Microbes cause infectious diseases. The microbes that commonly cause the opportunistic infections that accompany HIV infection are as follows:     Viruses: Cytomegalovirus, herpes simplex, herpes zoster, mol-luscum contagiosum     Bacteria: Mycobacterium avium-intracellulare, Mycobacterium tuberculosis (the cause of tuberculosis), Salmonella, Nocardia     Parasites: Toxoplasma gondii, Pneumocystis carinii, Cryptosporidium, Isospora     Fungi: Cryptococcus, Histoplasma, Candida albicans     Mycobacterium avium-intracellulare {MAI): MAI is related to the bacterium that causes tuberculosis, though it is not contagious and is more difficult to treat. In the late stages of HIV infection, infection with MAI is spread widely throughout many organs in the body. It can cause fever, pneumonia, diarrhea, hepatitis, and many other complications.     National Institutes of Health (NIH): The NIH is a federal organization located in Bethesda, Maryland, that funds scientific research. The NIH is the world’s largest research organization. With a budget of over $10 billion a year, the NIH is responsible for funding about a third of all research in the biomedical sciences, including research related to HIV infection, in the United States. Some of the research sponsored by NIH is intramural, that is, it is conducted by the approximately one thousand researchers inside NIH; most of the research is extramural, at universities and medical schools throughout the country. Extramural research grants are awarded on the basis of priority, as determined by expert review of proposals. The NIH is divided into fifteen different institutes, each with a different scientific specialty: the National Institute for Allergy and Infectious Diseases (MAID) is responsible for most of the research into HIV infection. The NIH is not related to the Centers for Disease Control (see above), except that both are federally funded agencies with somewhat different roles in combating HIV and other diseases.     Funding for research into HIV infection from sources other than NIH comes from other federal agencies (Department of Defense, National Science Foundation, Veterans Administration, and the Centers for Disease Control), pharmaceutical companies, local governments, and private foundations. Funding for this research escalated rapidly in the late 1980s until the total HIV research budget exceeded the funding for heart disease research at a time when heart disease was responsible for twenty times more deaths than AIDS was. Some view this as inappropriate, given the relative impact of the two; others feel AIDS research is underfunded, given its importance as a public health problem and as a prototypic disease for many other conditions.     Neuropathy: Neuropathy is an illness involving the nerves. Nerves are responsible for (among other things) the movement of muscles and the sensation of touch, including the sensation of pain. The symptoms of a neuropathy can therefore be weakness of a muscle or pain and tingling. In people with HIV infection, the most frequent symptoms of neuropathy are painful feet and legs.     Opportunistic infections: In all infectious diseases, the body’s defenses are, for a while, inadequate to control microbial invasion. Many microbes can cause disease in people who are otherwise healthy. Other microbes, however, are fairly hapless and can cause disease only in people whose immune defenses are weakened. These microbes are called opportunistic microbes because the microbe takes the opportunity offered by a weakened immune system to cause disease. The opportunistic microbes that most frequently infect people with HIV infection are summarized under Microbes.     Opportunistic tumors: Opportunistic tumors, like opportunistic infection, occur primarily in people with weakened immune systems. In people with HIV infection, the major opportunistic tumors are Kaposi’s sarcoma and certain types of lymphoma.     Oral hairy leukoplakia (OHL): The symptoms of oral hairy leukoplakia are white (leuko) patches (plakia) on the tongue and elsewhere in the mouth. It usually produces no symptoms, but may distort taste or cause pain. It is caused by the same virus that causes infectious mononucleosis. These patches often appear similar to those of thrush; in fact, oral hairy leukoplakia is often diagnosed when people who appear to have thrush do not respond to the usual treatment. It can also be diagnosed with a biopsy of the patches. Oral hairy leukoplakia seems to occur exclusively in people with HIV infection. It generally indicates progressive weakening of the immune system; without treatment for HIV, the first AIDS-defining diagnosis is likely in the next two or three years. The treatment for oral hairy leukoplakia is an antiviral drug called acyclovir.      P24 antigen test: The P24 antigen test is, like the PCR test (see Polymerase chain reaction, below) and a culture (see above) for HIV, a test that detects the presence of HIV in the blood. P24 is one of the several proteins (the protein with a molecular weight of 24,000) that make up HIV. An antigen is anything that causes the immune system to identify it as foreign and to manufacture antibodies against it. A P24 antigen test detects P24 and therefore HIV. Unlike PCR and HIV cultures, however, the P24 antigen test is not especially sensitive, and most people with HIV infection have tests for the P24 antigen that are negative.     Levels of P24 are highest both early and late in the disease; the numbers of HIV are likewise highest at the same times. Some physicians therefore suggest that the P24 antigen test might help track the course of the disease in people with HIV infection. That is, it might identify people with HIV infection who are likely to develop symptoms and who are most likely to transmit the virus to others; and it might help evaluate how people are responding to antiviral drugs.     Pentamidine: Pentamidine is a drug used to treat or prevent pneumocystis pneumonia. It can be taken by vein or delivered directly into the lungs as an aerosol, using a breathing machine. When given by vein, pentamidine often has such side effects as low blood pressure (causing fainting), low blood sugar, high blood sugar (diabetes), kidney failure, liver disease, low blood counts, or inflammation of the pancreas. These side effects are common when the drug is given by vein. They are rare or don’t occur at all when pentamidine is taken as an aerosol, since so little of the drug gets into the system. Nevertheless, intravenous treatment is usually preferred for people who have advanced Pneumocystis pneumonia.     Persistent generalized lymph adenopathy (PGL): A diagnosis of PGL means that lymph glands are swollen for at least one month and at two different sites of the body, not counting the groin area. PGL often occurs early in HIV infection and is generally of little consequence unless the symptoms have actually been caused by another disease. (See Lymph glands and Lymphadenopathy, above.)     Platelets: Platelets are the component of blood that facilitates clotting. The number of platelets is often low in people with HIV infection— sometimes so extremely low that the person is prone to bleeding. The cause of the low platelets may be HIV infection itself, or it may be the drugs that are used to treat people with HIV infection.     Pneumococcal vaccine: The most common cause of bacterial pneumonia in people without HIV infection is a bacterium called Streptococcus pneumoniae or pneumococcus. Pneumococcus is also a common cause of pneumonia in people with HIV infection. Pneumococcal vaccine is recommended for people, including people with HIV infection, who are especially prone to frequent or severe infections by pneumococcus. It is best to take this vaccine relatively early in the course of the disease when the immune system is strong.     Pneumocystis carinii: Pneumocystis carinii is a parasite that commonly causes lung infection and pneumonia in people with HIV infection. Pneumocystis carinii pneumonia (PCP) is the most frequent serious opportunistic infection in people with HIV infection. When there is no explanation for immune suppression other than HIV infection, Pneumocystis pneumonia is an AIDS-defining diagnosis. The symptoms are cough without sputum, shortness of breath, and fever. These symptoms usually evolve over a period of several days or, more commonly, weeks. The diagnosis is generally established by a chest x-ray or studies of lung function, combined with a microscopic examination of respiratory secretions to show the parasite. Treatment with several drugs—most commonly trimethoprim-sulfamethoxazole and pentamidine—is successful. Treatment is most successful when started relatively early in the course of the infection.     Pneumonia: Pneumonia is an infection of the lungs. The usual symptoms are cough, fever, and shortness of breath. The causes of pneumonia vary, and the treatment depends on the cause.     Polymerase chain reaction (PCR): Polymerase chain reaction is a very sensitive test, developed in the late 1980s, for detecting tiny quan-      ’ tities of HIV (see Retrovirus, below). Unlike the standard blood test for HIV infection which detects antibodies to HIV, the PCR detects HIV itself. The test is very accurate; over 95 percent of people with HIV infection will test positive on a PCR test. The great majority of people who take the standard antibody test for HIV infection need not take the PCR test. PCR is most useful when the results of the antibody test are ambiguous and in research studies. PCR is similar to a culture (see above) for HIV but is substantially less expensive. Neither PCR nor cultures for HIV, however, are available in most laboratories.     Progressive multifocal leukoencephalopathy: Progressive multifocal leukoencephalopathy is a viral infection deep in the brain that is found only in people with severely weakened immune systems, including, occasionally, people with HIV infection. Progressive multifocal leukoencephalopathy has a distinctive appearance on CAT or MRI scans of the brain, but a diagnosis can be established definitely only with a biopsy. The infection tends to be progressive, and no therapy is known to be effective.      Prophylaxis: Prophylaxis is treatment to prevent a disease, as opposed to treatment to eliminate a disease already present.      PWA: PWA is the abbreviation for people with AIDS. The   PWA Coalition is one of several national organizations that provides newsletters, lists of resources, and research updates to people with HIV infection and AIDS.     Pyrimethamine {Daraprim): Pyrimethamine is an antibiotic used to treat toxoplasmosis (see below). The full treatment usually combines pyrimethamine with a sulfa drug like sulfadiazine or clindamycin. Pyrimethamine is taken by mouth. The major side effect after prolonged use is anemia. To avoid anemia, another drug, leucovorin, is given at the same time. Other side effects include gastric intolerance, allergic reactions, and hepatitis. Many of these reactions are the result of the sulfa drug that is taken with pyrimethamine.      Research: See National Institutes of Health, above.      Retinitis: Retinitis means an inflammation (itis) of the retina, the layer of cells at the back of the eye that collects and send images to the brain. Retinitis usually causes some loss of vision. In people with HIV infection, the most common cause of retinitis is infection with cytomegalovirus (see above).     Retrovirus: Retroviruses are a type of virus. Retroviruses do not have DNA, the molecule that holds the code which cells use to reproduce themselves. Instead, retroviruses have RNA, a molecule that holds the retrovirus’s code, and an enzyme called reverse transcriptase, that turns RNA into DNA. When a retrovirus invades one of the cells of the body, it uses reverse transcriptase to turn its own RNA into DNA. This DNA then becomes part of the cell’s DNA, which in turn produces the retrovirus instead of the cell. Many different kinds of retroviruses infect many different kinds of animals. HIV is the most important retrovirus to infect humans; it causes disease in no other animal species.     Risk factor: A risk factor is a condition or behavior that makes it likely that a person with the risk factor will develop a condition—in this case, HIV infection. The major risk factors for HIV infection are needle-sharing with intravenous drug users and sexual contact with a person who has or may have been exposed to HIV. Another risk factor is having received blood products between 1978—when HIV infection was first known to exist in the United States—and May 1985, when the blood supply was first screened for HIV. Another risk factor is promiscuous or casual sexual contact without precautions. Still another is to be born to a woman with HIV infection: approximately 30 to 35 percent of women who are infected with HIV will pass the virus to the unborn infant. A minor risk factor is needlestick injuries in health care workers who care for people with HIV infection. Less than one percent of all people with HIV infection in the United States have no clearly defined risk factor, although many of these people either are too sick to provide adequate information or are providing information that is suspect.     Safer sex: Safer sex is a qualitative term. To be absolutely safe, sexual contact cannot involve an exchange of any body fluids—specifically, semen or vaginal secretions. The term safer sex recognizes the likelihood of human error and the inexactness of human knowledge. Safer sex refers to sexual intercourse using a condom and spermicide, or sexual practices that do not involve exchange of body fluids.     Seizure: A seizure is a convulsion, uncontrolled movements of the arms and legs accompanied by unconsciousness and inability to control urine or stool. The usual cause of seizures in people with HIV infection is an opportunistic infection or an opportunistic tumor of the brain, including toxoplasmic encephalitis, cryptococcal meningitis, or lymphoma. Less commonly, seizures are caused by an imbalance of electrolytes or by medications. Recurrent seizures can usually be controlled with drugs like Dilantin and phenobarbital. Anyone with recurrent seizures should be careful about his or her physical circumstances: be careful working on ladders, for instance, or driving. In many states it is illegal for a person with seizures to drive until seizures have been controlled for at least one year.     Seroconversion: The immune system usually takes several days or weeks to recognize a foreign substance like a virus and to produce antibodies to it. Six to twelve weeks (but sometimes years) after HIV enters the body, antibodies to HIV usually appear in the blood. Physicians call the appearance of antibodies in the blood seroconversion. That is, the result of a test for antibodies in the blood serum converts from negative to positive.     Shingles: Synonymous with herpes zoster (see above).     Sinusitis: The sinuses are air sacs next to the passageway from the nose. Sinusitis is an infection of the sinuses, usually as a result of a cold or an allergy. Sinusitis is common in everyone, and is especially common in people with HIV infection. The reason for this is obscure.  Symptoms are pus drainage from the nose, headache, face pain, and fever. The usual treatment is with antibiotics taken by mouth, such as trimethoprim-sulfamethoxazole, amoxicillin, erythromycin, cephalexin (or Keflex), ciprofloxacin (Cipro), or tetracycline. Some people do not respond to these drugs, and their sinuses need to be drained, a procedure done by a specialist called an otolaryngologist (ear, nose, and throat specialist).     Spinal tap: A spinal tap, also called a lumbar puncture, is a procedure for obtaining cerebrospinal fluid, the fluid that surrounds the brain and the spinal cord. The procedure involves inserting a needle into the middle of the back and into the meninges, a membrane that contains the cerebrospinal fluid. The cerebrospinal fluid is then analyzed for evidence of infection of the brain or spinal cord. Despite sounding unpleasant and risky, a spinal tap is a well-established medical procedure and is rarely associated with any important complications. The most common complaint is a headache following the spinal tap, a complaint made less likely by lying flat once the spinal tap is completed.     T-helper cells: Synonymous with T4 cells, T4 lymphocytes, and CD4 lymphocytes. (See CD4 cells, above).     T-suppressor lymphocytes: T-suppressor lymphocytes are another class of T lymphocytes (see Immune system, above). T-suppressor lymphocytes are synonymous with T8 cells, CD8 cells, and T8 lymphocytes. All T lymphocytes participate in the body’s defenses. T8 cells primarily regulate antibody formation by the B lymphocytes. The laboratory test called the T-cell subset analysis is a count of the various types of T lymphocytes. At one time, the proportion of the number of T4 cells to the number of T8 cells was believed to show where the person was in the course of HIV infection. This proportion was commonly referred to as the T-helperl/T-suppressor ratio. More recent work indicates that only the number of T4 cells (CD4 cells) is important, and counting T4 cells alone is usually far less expensive than doing the total subset analysis.     Thrombocytopenia: Thrombocytopenia is a low count (penia) of thrombocytes (or platelets), cells in the blood which facilitate clotting. The usual count of thrombocytes is 150,000 to 300,000 per milliliter of blood. Lower counts of 80,000 to 120,000 per milliliter are common in people with HIV infection. When the count is very low, from 5,000 to 25,000 per milliliter, bleeding problems may occur. People with HIV infection have thrombocytopenia because their bodies produce antibodies against their own platelets. Some people have no symptoms but must still be careful to avoid cuts or anything that could cause bleeding. Other people have excessive nosebleeds, excessive bleeding from cuts, and red spots the size of pinheads that come from tiny hemorrhages into the skin. Treatment is with drugs—corticosteroids (see above), AZT, gamma globulin given intravenously, alpha interferon—or with a splenectomy, the surgical removal of the spleen.     Thrush: Thrush is an infection of the mouth caused by the fungus Candida albicans. The symptoms are white patches along the gums, on the inside of the cheeks, or on the tongue. Thrush is extremely common in people with HIV infection, and is considered part of AIDS-related complex, or ARC (see above). Thrush is easily treated with nystatin, clotrimazole, ketoconazole, or fluconazole. .     Toxoplasmosis: Toxoplasmosis is an infection caused by the parasite Toxoplasma gondii. Toxoplasma gondii is found in cat excrement and in rare meat, both of which are the most common sources of infection. About 30 percent of all adults in the United States have Toxoplasma gondii in their bodies, but the majority are unaware of it. The parasite remains dormant (see Latency, above) and rarely causes disease unless the immune system is weakened. In people with HIV infection, the most common form of toxoplasmosis is an infection of the brain called toxoplasmic encephalitis.     Tuberculosis (TB): Tuberculosis is an infection, usually in the lungs, that is far more frequent in people with HIV infection than in the general population. The bacterium that causes TB can either be dormant (inactive TB) or active (active TB). In active TB, the usual symptoms are fever and cough. People with either active or inactive TB will have skin tests that are positive for TB. In people with HIV infection, the skin test is less reliable, especially in the later stages of HIV infection when the immune system is weakened. People with HIV infection and inactive TB should receive treatment—a drug called isoniazid—to prevent active TB. People with HIV infection and active TB should receive a combination of isoniazid and rifampin.     Vaccine: A vaccine is a drug, given by mouth or by an injection, that stimulates the immune system to form antibodies to some microbe. The polio vaccine, for instance, stimulates the immune system to form antibodies against the polio virus. These newly formed antibodies now protect the person against any subsequent exposure to that microbe. Some vaccines work better than others: with the polio vaccine, protection is nearly 100 percent; with the influenza vaccine, protection is about 70 percent. Vaccines for HIV infection are being tested in people with and without HIV infection.     Varicella zoster: Varicella zoster is the virus that causes chickenpox (varicella) and herpes zoster (shingles). See Herpes zoster, above.     Virus: A virus is a tiny microbe that, unlike bacteria, can neither survive nor reproduce unless it lives in a cell. HIV is a virus that lives in CD4 lymphocytes in humans.     Wasting: Wasting is the term given—somewhat unfortunately—to the weight loss and malnutrition that often accompany HIV infection. The causes of wasting vary; they may include opportunistic infections and tumors, or they may simply be the result of HIV infection itself. In some cases, wasting is accompanied by diarrhea; the condition is then called the diarrhea-wasting syndrome. Wasting can be an AIDS-defining diagnosis: according to the criteria of the Centers for Disease Control, an unexplained loss of at least 10 percent of the usual body weight is diagnostic of AIDS.     Western blot: The Western blot is a test for specific antibodies, in this case, for antibodies to HIV. (See above, under ELISA.)*268\191\2*

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