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AIDS: Advanced infection with HIV, marked by certain conditions that do not occur in people with healthy immune systems, or by a decrease in CD4 cells below a count of 200.

anti-retroviral: Any drug that interferes with the ability of HIV to make new copies of itself inside infected cells.

CD4 cells: White blood cells that help direct the body's infection-fighting cells. They're named after the CD4 molecules they carry on their surface. Sometimes they're called CD4 lymphocytes or T helper cells.

CD4 count: The number of CD4 cells in a tiny drop of blood (a microliter, or about 1/5000th of a teaspoon). Because HIV attacks CD4 cells, their number falls as the infection gets more serious. In most healthy people without HIV infection, the CD4 count is around 1000. When the CD4 count falls below 200, a person infected with HIV is said to have AIDS and the danger of other serious infections and cancers becomes great.

enzyme: A kind of protein that causes chemical changes inside cells.

genetic: Having to do with genes, which are tiny segments of chemicals inside all viruses and cells. Genes carry the information needed to make proteins that perform basic functions (such as making new copies of viruses).

HIV: The human immunodeficiency virus. It attacks infection-fighting blood cells (CD4 cells) and other cells and causes AIDS.

immune system: The network of body organs and cells that recognizes and fights off infections and other "foreign" invaders.

immunotherapy: Any therapy that takes advantage of certain proteins in the body that either slow down or speed up the activity of immune system cells, such as CD4 cells. Also called immune-based therapy. Researchers are studying ways to combine immunotherapies with anti-retroviral drugs.

non-nucleoside: Short name for non-nucleoside reverse transcriptase inhibitor.

non-nucleoside reverse transcriptase inhibitor: Drugs that limit HIV's ability to make new copies of itself by interfering with the HIV enzyme reverse transcriptase. Often simply called non-nucleosides or NNRTIs. They differ from the group of drugs called nucleoside analog reverse transcriptase inhibitors in how their molecules are linked together and in how they interfere with reverse transcriptase.

nucleoside: Short name for nucleoside analog reverse transcriptase inhibitor.

nucleoside analog reverse transcriptase inhibitor: Drugs that limit HIV's ability to make new copies of itself by interfering with the HIV enzyme reverse transcriptase. Often simply called nucleosides.They differ from the group of drugs called non-nucleoside reverse transcriptase inhibitors in how their molecules are linked together and in how they interfere with reverse transcriptase.

protease: An enzyme that HIV uses to cut long chains of HIV proteins and enzymes into smaller pieces (Figure 1). HIV needs these smaller pieces to make active new copies of itself.

protease inhibitor Drugs that limit HIV's ability to make new copies of itself by interfering with the HIV enzyme protease.

proteins: Complex compounds that are a major part of cells in all plants and animals. Inside cells, HIV can make certain proteins that it needs to create new copies of itself.

resistance: The ability of HIV to change its genetic makeup in a way that makes drugs ineffective against it.

retrovirus: Any virus, such as HIV, that stores its genetic information in an acid called RNA instead of the one called DNA. Herpesviruses like cytomegalovirus (CMV), for example, store their genetic information in DNA.

reverse transcriptase An enzyme HIV uses to get its genetic message inside the cell's command center, its nucleus (Figure 1).

viral load: The amount of virus floating in the liquid part of blood (outside of blood cells). Tests that measure viral load are being used to help decide when to start therapy against HIV and when a therapy is no longer working well.

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