Etiology and Epidemiology

Beginning in the late 1970s, physicians in New York and California reported the increasing occurrence of a rare type of cancer, Kaposi's sarcoma, and a variety of infections including Pneumocystis pneumonia among previously healthy young homosexual men. Because of the unusual character of these diseases, which are typically associated with a failure of the immune system, epidemiologists began to search for characteristics that might link these cases. AIDS was first formally described in 1981, although it now appears that the virus that causes the disease must have been silently spreading in a number of populations during the previous decade. Early epidemiological studies suggested that homosexual men, recipients of blood transfusions and blood products (especially hemophiliacs), and intravenous drug users were at greatest risk for the disease. For this reason, research focused on a search for a common infectious agent that could be transmitted sexually or through blood. This research led to the identification in 1983, in French and American laboratories, of a previously undescribed human retrovirus. Officially named HTV-1 for human immunodeficiency virus, this organism is an RNA retrovirus. Although the biological and geographic origins of the organism remain obscure, the AIDS epidemic appears to mark the first time this organism has spread widely in human populations. There is no evidence for casual transmission of HIV.

Following the identification of HIV-1, tests to detect antiviral antibodies to the virus were devised in 1984. Although these tests do not detect the virus itself, they are generally effective in identifying infection because high levels of antibody are produced in most infected individuals. The enzyme-linked immunosorbant assay (ELISA), followed by Western blot testing, has made possible the screening of donated blood to protect the blood supply from HIV, as well as testing for epidemiological and diagnostic purposes.

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