NUMBERS: The Centers for Disease Control reports that, as of Oct. 29, 6,620 Americans had contracted AIDS and 3,089 of them had died. WHO GETS IT: The groups at highest risk are homosexual or bisexual men, who account for three-fourths of the total cases, intravenous drug users, Haitians, hemophiliacs and their sexual contacts. Blood tranfusions account for 1 percent of the total cases. Most victims are white males under 40, but a small number of women and children also have contracted the disease.

CAUSE: An unusual virus called a retrovirus was identified by three research groups earlier this year. Rather than acting rapidly to cause acute infection, it can slowly attack the body's natural immune system, making some of its victims vulnerable to life-threatening infections or cancer.

SPREAD: AIDS is not contagious like the common cold. Researchers believe it is spread only through intimate contact with bodily fluids, particularly through sexual contact or contact with blood.

DETECTION: A new blood test may become commercially available next year to detect whether a person has the AIDS virus. It will not say whether a person will get the disease. Other lab tests can measure changes in the immune system.

TREATMENT: AIDS is incurable, but doctors are increasingly able to treat the complications. Research is under way on an antiviral drug. An attempt to develop a vaccine is just beginning.