are socially and sexually engaged with each other. These intimate acts take place between specific people, acting in specific places. The central problem in AIDS epidemiology, from the perspective of the social and behavioral sciences, is to describe the connections between individual acts and social settings that lead to the spread of infection. On the basis of the careful delineation of the social structure of viral transmission, scientists can develop targeted strategies for prevention campaigns.

Epidemic surveillance—numbers of AIDS cases and HIV infection—is an essential first element for delineating the social and geographic structure of the epidemic. Those basic data provide the framework for more detailed analysis of the specific social settings and interpersonal interactions that will promote or contain further spread of infection, and have been useful for estimating the magnitude of the AIDS epidemic. As with all estimates, however, the number of AIDS cases is an imperfect reflection of the magnitude of the epidemic. AIDS cases reflect only the numbers of people with advanced illness. AIDS is characteristic of the end stage of HIV disease, and may follow a period of latency or milder illness lasting, on average, about 10 years. AIDS cases thus reflect the patterns of transmission and infection that occurred at some time in the past. The criteria for AIDS, originally established in 1981, have been altered in 1985, 1987, and 1993 to reflect the information accumulated about the natural history of HIV disease. Finally, AIDS case reporting is incomplete. Although in the United States a vast majority of cases are reported, this is not true in many parts of the world. Despite these limitations, AIDS case reporting has been a central tool for examining the epidemic, and has revealed much about its spread over the past 13 years.

In the United States, the epidemic has disproportionately affected men, who constituted more than 85 percent of the adults diagnosed with AIDS as of December, 1993. Early in the epidemic, the predominance of cases among men who had sex with men gave rise to the stereotype that AIDS was a "gay" disease. However, this stereotype has been challenged as the list of groups at risk for contracting HIV has expanded to include injection drug users, their sexual partners, infants born to mothers infected with HIV and, in very rare cases, those exposed to blood or blood products by occupational activities or transfusion.