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Today AIDS has become a major cause of morbidity and mortality in the U.S. Indeed, it has become the leading cause of death in the country among people with hemophilia and users of

illegal intravenous (IV) drugs. Moreover, nation-wide morbidity and mortality rates will increase in the next few years as some of the one to 1.5 million Americans who are already infected with the human immunodeficiency virus (HIV) develop AIDS. Most of those affected in the near future will be either homosexual men or IV drug abusers, and a significant proportion of them will be blacks and Hispanics. Yet, given the fact that the virus is transmitted through sexual contact, through the traces of blood in needles and other drug paraphernalia and from mother to newborn infant, one can envision many possible chains of infection, which leave no segment of the U.S. population completely unaffected by the threat of AIDS.

The discovery of the epidemic, the enumeration of the varied manifestations of HIV infection and the analysis of the circumstances that made it possible for such an infection to spread have been missions assigned to epidemiology: the study of the occurrence and distribution of disease as well as its control in a given population. Epidemiologists monitor mortality and morbidity rates associated with HIV infection and AIDS; they also make predictions of likely changes in HIV infection rates in the course of time.

Most important, by carrying out studies to define the ways HIV is transmitted from person to person, epidemiologists can identify the population groups that are at greatest risk of acquiring AIDS and thereby develop strategies for the prevention and control of the disease — strategies that are independent of the development of an effective vaccine or therapy. Indeed, determining the risk factors for AIDS enabled the U.S. Public Health Service and other groups to issue recommendations for the prevention of AIDS as early as 1983, a full year before HIV was firmly identified and two years before laboratory tests to detect the presence of the virus became widely available.

To carry out all these tasks epidemiologists depend on surveillance: the gathering of high-quality, consistent and interpretable data on a disease or an infection. Surveillance data are routinely compiled from reports filed with state and local health departments that are then forwarded to the U.S. Centers for Disease Control (CDC).

Because the disease appeared to be transmitted through the exchange of blood or by sexual contact, most investigators were convinced by late 1982 that the cause of AIDS was an infectious agent (most likely a virus) and not the result of exposure to toxic substance^ or other environmental or genetic factors. The infection hypothesis was finally confirmed when HIV was isolated by Luc Montagnier and his colleagues at the Pasteur Institute in Paris and by Robert C. Gallo and his colleagues at the National Cancer Institute.

Soon after the discovery of the AIDS agent a laboratory test was developed to detect antibodies to HIV in the blood. A positive result in a test of a person's 'blood sample was a, reliable sign that the person was infected with the virus. Such a serological test made it possible to detect HIV infection in people who showed no clinical symptoms, and to confirm clinical diagnoses of AIDS and other HIV-related conditions. It also made it possible to measure directly the prevalence of HIV infection (the number of infected people in a given population at a given time) and its incidence (the number of new infections occurring within a defined period in a specific population). Most important, perhaps, was the fact that the national supply of donated blood could now be screened, so that additional cases of AIDS due to blood transfusions and contaminated blood products could be avoided.

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