Epidemiologic observations indicate a pandemic of a highly pathogenic, apparently new human infection. Based on current estimates of the prevalence of infection, the cumulative incidence of disease among infected persons and the duration of latency, hundreds of thousands of cases of HIV disease will probably be diagnosed in the United States in the next decade. The health care system must mobilize to deal with this disease. Most importantly, the transmission of HIV infection is largely preventable. Transmission requires either intimate sexual exposure, invasive exposure to infected blood, or perinatal exposure. The virus has been found in blood, semen, vaginal secretions, tears, and saliva, but present epidemiological evidence suggests that only the first three sources of virus are important. As people guard against the transmission of the virus in these areas of exposure, the spread of the disease can be curtailed. The public health community should give priority to educating the public about how to prevent the transmission of the virus. 56 references.