The clinical manifestations of HIV-1 infection are numerous, involving multiple organ systems and both infectious and neoplastic disease processes. From the beginning of the AIDS epidemic there was a need for a reportable case definition that was sensitive enough to monitor the epidemic but specific enough to exclude most cases that were not due to the then unknown agent. The first surveillance definition of an AIDS case used by the CDC was published in Morbidity and Mortality Weekly Roper (1) before HIV was identified, and therefore did not use HIV test results. AIDS was defined as “a case of a disease at least moderately predictive of a defect in cell-mediated immunity occurring in a person with no known cause for diminished resistance to that disease.”

This general definition of AIDS included a list of specific diseases and qualifications as well as a list of disqualifying conditions that were considered possible causes of non-AIDS-related diminished resistance. Many of the AIDS-defining diseases were known to occur infrequently in persons who were not HIV-infected, while other diseases occurring in HIV-positive persons were left out of the case definition because they are common in HIV-negative persons. The surveillance definition is therefore a compromise that has required revision over time as more information has become available.