Distribution and Incidence

Although identified only in 1981, AIDS can now be found throughout the world. Spread by sexual contact, by infected blood and blood products, and perinatally from mother to infant, AIDS had been reported in 138 countries by July 1988, according to the World Health Organization. Since HIV infection precedes the development of AIDS, often by as many as 7 to 11 years, the precise parameters of the epidemic have been difficult to define. Estimates suggest that worldwide between 5 and 10 million individuals were infected with the virus by the end of 1988. At that time, in the United States more than 80,000 cases of AIDS had occurred, and nearly 50,000 of these individuals had died. Projections by the U.S. Public Health Service Centers for Disease Control estimate that between 1.5 and 2.0 million Americans are infected with HIV but are currently asymptomatic. Although the "cofactors" that may determine the onset of symptoms remain unknown, all current evidence suggests that HIV-infected individuals will eventually develop AIDS.

Researchers have identified three epidemiological patterns of HIV transmission, which roughly follow geographic boundaries. Pattern I includes North America, Western Europe, Australia, New Zealand, and many urban centers in Latin America. In these industrial, highly developed areas, transmission has been predominantly among homosexual and bisexual men. Since the introduction of widespread blood screening, transmission via blood now occurs principally among intravenous drug users who share injection equipment in these areas. Although there is no evidence of widespread infection among the heterosexual population in these countries, het erosexual transmission of the virus from those infected via intravenous drug use has increased, leading to a rise in pediatric cases resulting from perinatal transmission.

Within the United States the distribution of AIDS cases has been marked by a disproportionate representation of the poor and minorities. As the principal mode of transmission has shifted to intravenous drug use, AIDS has increasingly become an affliction of the urban underclass, those at greatest risk for drug addiction. Serum surveys reveal that 50 percent or more of the intravenous drug users in New York City are infected with HIV. Blacks and Hispanics, who comprise 20 percent of the U.S. population, accounted in 1988 for more than 40 percent of all AIDS cases. Women, who account for more than 10 percent of new AIDS cases, are typically infected by intravenous drug use or by sexual contact with a drug user; in 70 percent of all infected newborns, transmission can be traced to drug use. By 1991, between 10,000 and 20,000 children in the United States are expected to have symptomatic HIV infections.

In pattern II countries, comprised of sub-Saharan Africa and, increasingly, Latin America, transmission of HIV occurs predominantly through heterosexual contact. In some urban areas in these countries, up to 25 percent of all sexually active adults are reported to be infected, and a majority of female prostitutes are seropositive. In addition, transfusion continues to be a mode of transmission because universal screening of blood is not routine. Unsterile injections and medical procedures may also be contributing to the spread of infection. In these areas, perinatal transmission is an important aspect of the epidemic; in some urban centers at least 5 to 15 percent of pregnant women have been found to be infected.

Pattern III countries, which include North Africa, the Middle East, Eastern Europe, Asia, and the Pacific, have thus far experienced less morbidity and mortality from the pandemic. Apparently, HIV-1 was not present in these areas until the mid-1980s; therefore, fewer than 1 percent of all cases have been found in pattern ni countries. Infection in these areas has been the result of contact with infected individuals from pattern I and II countries, or importation of infected blood. The nature of world travel, however, has diminished the significance of geographic isolation as a means of protecting a population from contact with a pathogen.

In 1985, a related virus, HIV-2, was discovered in West Africa. Although early reports have suggested that HIV-2 may be less pathogenic, the natural his tory of infection with this agent remains unclear, as does its prevalence.

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