AIDS is a chronic disease that damages, and ultimately destroys, the immune system. Though HIV causes AIDS, many patients who test positive for the virus have not progressed to AIDS. According to the national Centers for Disease Control and Prevention, an AIDS diagnosis requires a positive confirmed blood test for HIV antibodies and at least one of the following:

* an opportunistic infection such as pneumocystis pneumonia
* an AIDS-related cancer, severe wasting, or dementia
* a reduction in the amount of the helper T cells--also called CD4 cells--that play a critical role in proper functioning of the immune system to below a count of 200 (healthy people usually have a helper T cell count between 600 and 1,000).

HIV depends on the cells it infects to make new copies of itself. The copies then infect other cells, spreading the virus. HIV destroys CD4 cells, and when the level of these white blood cells drops, the immune system weakens, allowing microorganisms that don't harm people with normal immune responses to cause serious infections in those with HIV.

HIV is transmitted primarily by sex (anal, vaginal or oral sex with an infected partner), by injections (sharing contaminated needles for drug use or accidental piercing with a contaminated needle), or from infected mother to child through pregnancy or breast-feeding.

Some HIV-infected patients progress to AIDS quickly while others can remain healthy for 10 years or more. Between initial infection and full-blown disease, a middle phase called symptomatic HIV infection, or AIDS-related complex (ARC), occurs, prompting symptoms such as weight loss, diarrhea, and swollen lymph glands.

Scientists have recently discovered clues to why some patients develop AIDS quickly. In a study published last March in the journal Science, National Cancer Institute researchers found that inherited genes may set the clock for AIDS progression. Certain gene patterns tend to stave off AIDS, while others promote it. The researchers say the study may help lead to an AIDS-preventive vaccine or improved therapies against the virus.

With the emergence of AIDS, FDA put into place a program in the late 1980s that allows promising therapies for life-threatening illnesses to be approved conditionally before all necessary studies are completed. A key goal is to make treatments available to desperately ill patients who might have to wait years under the formal clinical trial and drug approval system for the same drug to be marketed.

Under the agency's accelerated approval regulations, a drug can be marketed without studies that show direct effects on clinical disease progression or death. Instead, FDA relies on "surrogate markers," such as viral load, which are laboratory measurements intended to reliably predict a drug's ultimate clinical benefits.

FDA has three requirements for accelerated approval:

* The surrogate must have a "reasonable certainty" of predicting actual future clinical benefit.
* The drug's sponsor must complete postmarketing studies, providing the required data to verify the drug's clinical benefit.
* The sponsor must prove clinical benefit in a timely manner or FDA will revoke the accelerated approval.