Measuring HIV prevalence (the number of infections at a point in time) and incidence (the number of new infections over time) is critical for monitoring the course of the epidemic. Descriptions of prevalence and incidence of HIV infection provide a more complete picture of the magnitude of the epidemic than do AIDS case reports, because they do not rely on presentation of end-stage disease. Efforts to determine HIV prevalence have been based on a range of seroprevalence studies—small cross-sectional studies and large-scale surveys. The most difficult challenge in conducting seroprevalence studies is the selection of the sample. Samples drawn from the general population, such as household surveys, will include many at low risk for infection, but few at highrisk for infection. Conversely, samples chosen to select those at higher risk cannot be generalized to the population at large. As a partial solution to this problem, surveys have been conducted among highrisk subgroups as well as in the general population. The combined insights provided by this ''family of surveys" have guided estimates of seroprevalence.

CDC has conducted seroprevalence surveys of sex workers, drug users (in and out of treatment), patients in STD clinics and sentinel hospitals, childbearing women, and newborn infants. The surveys have been of two types: blinded surveys, in which the serum cannot be connected to the donor, and unblinded surveys, in which people provide consent for their blood to be tested and the results can be linked to the donor. All branches of the military have conducted HIV testing on new recruits as well as enlisted personnel. The Jobs Corps has conducted HIV testing of its applicants. The National Center for Health Statistics has conducted a national household study of seroprevalence. Local studies of the general population have been conducted in Dallas, San Francisco, and Chicago. Studies of special populations, such as psychiatric patients, homeless psychiatric patients, runaway adolescents, and prisoners, have been conducted in localities throughout the United States.

These seroprevalence studies demonstrate that HIV infection spread rapidly through the large, tightly knit, geographically concentrated networks of gay men and injection drug users that occurred in certain cities. The San Francisco Cohort, a study that began in the 1970s as a study of the hepatitis vaccine, was able to document retrospectively that substantial spread of HIV had transpired before the first cases were officially recognized in 1981.

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