A study in which participants are randomly (i.e., by chance) assigned to one of two or more treatment arms or regimens of a clinical trial. Occasionally placebos are utilized. Randomization minimizes the differences among groups by equally distributing people with particular characteristics among all the trial arms.

A specific blood rest for syphilis.

Reduction in a pathogen's sensitivity to a particular drug. Resistance is thought to result usually from a genetic mutation. In HIV, such mutations can change the structure of viral enzymes and proteins so that an antiviral drug can no longer bind with them as well as it used to. Resistance detected by searching a pathogen's genetic makeup for mutations thought to confer lower susceptibility is called "genotypic resistance." Resistance that is found by successfully growing laboratory cultures of the pathogen in the presence of a drug is called "phenotypic resistance."

Inflammation of the retina, linked in AIDS to cytomegalovirus (CMV) infection. Untreated, it can lead to blindness.

A type of virus that, when not infecting a cell, stores its genetic information on a single-stranded RNA molecule instead of the more usual double-stranded DNA. HIV is an example of a retrovirus. After a retrovirus penetrates a cell, it constructs a DNA version of its genes using a special enzyme called reverse transcriptase. This DNA then becomes part of the cell's genetic material.

This enzyme of HIV and other retroviruses converts the single-stranded viral RNA into DNA, the form in which the cell carries its genes. Some antiviral drugs approved by the FDA for the treatment of HIV infection work by interfering with this stage of the viral life cycle. They are referred to as nucleoside reverse transcriptase inhibitors (NRTIs).

A class of anti-retroviral drugs.

A nucleic acid, found mostly in the cytoplasm of cells (rather than the nucleus) that is important in the synthesis of proteins. The amount of RNA varies from cell to cell. RNA, like the structurally similar DNA, is a chain made up of subunits called nucleotides. Some viruses, such as HIV, carry RNA instead of the more usual genetic material DNA. See Cytoplasm; Retrovirus.