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CANDIDA: Yeast-like fungi commonly found in the normal flora of the mouth, skin, intestinal tract and vagina, but can become clinically infectious in immune compromised people. See also Fungus.

CARCINOGEN: Any cancer-producing substance.

CBCT: See Community-Based Clinical Trial.

CBO: See Community-Based Organization.

CDC: See Centers for Disease Control and Prevention.

CD8 (T8) CELLS: A protein embedded in the cell surface of suppressor T lymphocytes. Also called cytotoxic T cells. See also CD Nomenclature; CD4 (T4) or CD4+ Cells; T Cells.

CD4 (T4) or CD4+ CELLS: 1. White blood cells killed or disabled during HIV infection. These cells normally orchestrate the immune response, signaling other cells in the immune system to perform their special functions. Also known as T helper cells. 2. HIV's preferred targets are cells that have a docking molecule called cluster designation 4 (CD4) on their surfaces. Cells with this molecule are known as CD4-positive (or CD4+) cells. Destruction of CD4+ lymphocytes is the major cause of the immunodeficiency observed in AIDS, and decreasing CD4+ lymphocyte levels appear to be the best indicator of morbidity in these patients. Although CD4 counts fall, the total T-cell level remains fairly constant through the course of HIV disease, due to a concomitant increase in the CD8+ cells. The ratio of CD4+ to CD8+ cells is therefore an important measure of disease progression. See also CD Nomenclature; CD8 (T8) Cells; Immunodeficiency.

CELL LINES: Specific cell types artificially maintained in the laboratory (i.e., in vitro) for scientific purposes.

CELL-MEDIATED IMMUNITY (CMI): The branch of the immune system in which the reaction to foreign material is performed by specific defense cells (i.e., killer cells, macrophage and other white blood cells) rather than antibodies.

CELLULAR IMMUNITY: See Cell-Mediated Immunity.

CENTERS FOR DISEASE CONTROL AND PREVENTION (CDC): A Public Health Service agency responsible (among others) for assessing the status and characteristics of the AIDS epidemic and the prevalence of HIV infections. CDC supports the design, implementation and evaluation of prevention activities, and maintains various HIV/AIDS information services, such as the CDC National AIDS Clearinghouse.

CENTRAL NERVOUS SYSTEM (CNS): Composed of the brain, spinal cord and its coverings (meninges).

CENTRAL NERVOUS SYSTEM (CNS) DAMAGE: (By HIV infection). Although monocytes and macrophages can be infected by HIV, they appear to be relatively resistant to killing. However, these cells travel throughout the body and carry HIV to various organs, especially the lungs and the brain. People infected with HIV often experience abnormalities in the central nervous system. Investigators have hypothesized that an accumulation of HIV in brain and nerve cells or the inappropriate release of cytokines or toxic byproducts by these cells may be to blame for the neurological manifestations of HIV disease. See also Cytokines; Macrophage; Monocyte.

CEREBRAL: Pertaining to the cerebrum, the main portion of the brain.

CERVICAL CANCER: A neoplasm of the uterine cervix that can be detected in the early curable stage by the Papanicolaou (Pap) test. See also Cervical Dysplasia; Cervix; Pap Smear.

CERVIX: The part of the uterus that protrudes into the cavity of the vagina. See also Uterus.

CHALLENGE: In vaccine experiments, the exposure of an immunized animal to the infectious agent.

CHEMOTHERAPY: The treatment, mostly of cancer, by the use of a series of cytotoxic drugs that attack cancerous cells. This treatment commonly has adverse side effects that may include the temporary loss of the body's natural immunity to infections, loss of hair, digestive upset and a general feeling of illness. Although unpleasant, the adverse effects of treatment are tolerated considering the life-threatening nature of the cancers usually treated by chemotherapy. See also Cytotoxic.

CLINICAL: Pertaining to or founded on observation and treatment of patients, as distinguished from theoretical or basic science.

CLINICAL ALERT: A mechanism, adopted by the National Institutes of Health in onjunction with the editors of several biomedical journals, for urgent cases in which timely and broad dissemination of results of clinical trials could prevent morbidity (sickness) and mortality (death). The Clinical Alert does not become a barrier to subsequent publication of the full research paper. Clinical Alerts are widely distributed electronically through the National Library of Medicine and through standard mailings.

CLINICAL LATENCY: The state or period of an infectious agent, such as a virus or bacterium, living or developing in a host without producing clinical symptoms. As related to HIV infection: Although infected individuals usually exhibit a period of clinical latency with little evidence of disease, the virus is never truly latent. Even early in the disease, HIV is active within lymphoid organs where large amounts of virus become trapped in the FDC network. Surrounding germinal centers are areas rich in CD4+ T cells. These cells increasingly become infected and viral particles accumulate both in infected cells and as free virus. See also CD4 (T4) or CD4+ Cells; Lymphoid Organs.

CLINICAL PRACTICE GUIDELINES: Standards for physicians to adhere to in prescribing care for a given condition or illness.

CLINICAL TRIAL: A carefully designed and executed investigation of the effects of a drug (or vaccine) administered to human subjects. The goal is to define the clinical efficacy and pharmacological effects (toxicity, side effects, incompatibilities or interactions) of the drug. The US government, through the Food and Drug Administration, requires strict testing of all new drugs and vaccines prior to their approval for use as therapeutic agents.

CMV: See Cytomegalovirus.

COFACTORS: 1. Substances, microorganisms or characteristics of individuals that may influence the progression of a disease or the likelihood of becoming ill. 2. A substance, such as a metallic ion or coenzyme, that must be associated with an enzyme for the enzyme to function. 3. A situation or activity that may increase a person's susceptibility to AIDS. Examples of such cofactors are other infections, drugs and alcohol use, poor nutrition, genetic factors and stress.

COHORT: In epidemiology, a group of individuals with some characteristics in common.

COMMUNITY-BASED CLINICAL TRIAL (CBCT): A clinical trial conducted primarily through primary-care physicians rather than academic research facilities.

COMMUNITY-BASED ORGANIZATION (CBO): A locally based service organization that provides social services at the community level.

COMPASSIONATE USE: A method of providing experimental drugs to very sick patients who have no other treatment options. Often, case-by-case approval must be obtained from the Food and Drug Administration for "compassionate use" of a drug.

CONCOMITANT DRUGS: Drugs that are taken together. Certain concomitant medications may have adverse interactions.

CONTAGIOUS: Any infectious disease capable of being transmitted by casual contact from one person to another. Casual contact can be defined as normal day-to-day contact between people at home, school, work or in the community. A contagious infection (e.g., a common cold) can be communicable by casual contact; an infectious infection, on the other hand, is communicable by intimate contact such as sex. AIDS is infectious, not contagious.

CONTRAINDICATION: A specific circumstance when the use of certain treatments could be harmful.

CONTROL: A standard against which experimental observations may be evaluated. In clinical trials, one group of patients is given an experimental drug, while another group (i.e., the control group) is given either a standard treatment for the disease or a placebo. See also Placebo.

CRYPTOSPORIDIOSIS: An opportunistic infection caused by the protozoan parasite Cryptosporidium parvum.

CUTANEOUS: Of, pertaining to or affecting the skin.

CYTOMEGALOVIRUS (CMV): A herpes virus that is a common cause of opportunistic diseases in people with AIDS and other people with immune suppression. While CMV can infect most organs of the body, people with AIDS are most susceptible to CMV retinitis (disease of the eye) and colitis (disease of the colon). See also Cytomegalovirus (CMV) Retinitis.

CYTOMEGALOVIRUS (CMV) RETINITIS: Most adults in the US have been infected by cytomegalovirus, although the virus usually does not cause disease in healthy people. Because the virus remains in the body for life, it can cause disease if the immune system becomes severely damaged by disease or suppressed by drugs. CMV retinitis is an eye disease common among people who are infected with HIV. Without treatment, people with CMV retinitis can lose their vision. CMV infection can affect both eyes and is the most common cause of blindness among people with AIDS.