DATABASE: An organized compilation of information, usually maintained in a computer system.

DDC: Dideoxycytidine (zalcitabine, HIVID), a nucleoside analog drug that inhibits the replication of HIV. See also Nucleoside Analog.

DDI: Dideoxyinosine (didanosine, Videx), a nucleoside analog drug that inhibits the replication of HIV. See also Nucleoside Analog.

DEMENTIA: Chronic intellectual impairment (i.e., loss of mental capacity) with organic origins that affects a person's ability to function in a social or occupational setting. See also AIDS Dementia Complex.

DENDRITIC CELLS: Patrolling immune system cells that may begin the HIV disease process by carrying the virus from the site of the infection to the lymph nodes, where other immune cells become infected. Dendritic cells travel through the body and bind to foreign invaders-such as HIV-especially in external tissues, such as the skin and the membranes of the gut, lungs and reproductive tract. They then ferry the foreign substance to the lymph nodes to stimulate T cells and initiate an immune response. In laboratory experiments, the dendritic cells that carry HIV also bind to CD4+ T cells, thereby allowing HIV to infect the CD4+ T cells. CD4+ T cells are the critical immune system cells targeted by HIV and depleted during HIV infection. See also CD4 (T4) or CD4+ Cells; Lymph Nodes; T Cells.

D4T: (Also known as Stavudine and Zerit). d4T is a dideoxynucleoside pyrimidine analog (2'3'-didehydro-3'-deoxythymidine). Like other nucleoside analogs, d4T inhibits HIV replication by inducing premature viral DNA chain termination. d4T has been approved for patients with advanced HIV infection intolerant to or failing other antiretroviral drugs. See also Nucleoside Analog.

DIAGNOSIS: The determination of the presence of a specific disease or infection, usually accomplished by evaluating clinical symptoms and laboratory tests.

DIARRHEA: Uncontrolled, loose and frequent bowel movements. In the United States, almost all people with AIDS develop diarrhea at some time in the course of their disease. Severe or prolonged diarrhea can lead to weight loss and malnutrition. The excessive loss of fluid that may occur with AIDS-related diarrhea can be life-threatening. There are many possible causes of diarrhea in people who have AIDS. The most common infectious organism causing AIDS-related diarrhea include cytomegalovirus (CMV); the parasites Cryptosporidium, Microsporidia and Giardia lamblia; and the bacterium Mycobacterium avium-inracellulare (MAC). Other bacteria and parasites that cause diarrheal symptoms in otherwise healthy people may cause more severe, prolonged or recurrent diarrhea in people with HIV or AIDS. See also Cytomegalovirus; Giardiasis; Microsporidiosis; Mycobacterium Avium Complex.

DNA: (Deoxyribonucleic Acid). 1. The molecular chain found in genes within the nucleus of each cell, which carries the genetic information that enables cells to reproduce. 2. DNA is the principal constituent of chromosomes, the structures that transmit hereditary characteristics. The amount of DNA is constant for all typical cells of any given species of plant or animal (including humans), regardless of the size or function of that cell. Each DNA molecule is a long, two-stranded chain made up of subunits, called nucleotides, containing a sugar (deoxyribose), a phosphate group and one of four nitrogenous bases: adenine (A), guanine (G), thymine (T) and cytosine (C). In 1953 J.D. Watson and F.H. Crick proposed that the strands, connected by hydrogen bonds between the bases, were coiled in a double helix. Adenine bonds only with thymine (A-T or T-A) and guanine only with cytosine (G-C or C-G). The complementarity of this bonding ensures that DNA can be replicated (i.e., that identical copies can be made in order to transmit genetic information to the next generation).

DORMANCY: See Latency.

DOSE-RANGING STUDY: A clinical trial in which two or more doses of an agent (such as a drug) are tested against each other to determine which dose works best and is least harmful. See also Clinical Trial.

DOUBLE-BLIND STUDY: A clinical trial design in which neither the participating individuals nor the study staff know which patients are receiving the experimental drug and which are receiving placebo or another therapy. Double-blind trials are thought to produce objective results, since the doctor's and patient's expectations about the experimental drug do not affect the outcome. See also Clinical Trial; Placebo.

DRUG-DRUG INTERACTION: A modification of the effect of a drug when administered with another drug. The effect may be an increase or a decrease in the action of either substance, or it may be an adverse effect that is not normally associated with either drug.