of heterosexual cases has also increased over the years (from 1 percent in 1981 to 9 percent in 1993), and women are disproportionately affected in this latter category (of all new AIDS cases reported in 1993, heterosexual contact accounted for 4 percent of AIDS cases among men and 37 percent among women). Injection drug use has been tied to the majority of AIDS cases among heterosexuals and is a major risk factor associated with pediatric cases. Furthermore, the rate of AIDS cases linked to injection drug use is disproportionately high for African Americans and Hispanics.

Injection drug users represent a sizable population. Estimates of current injection drug users in the United States range from 1.1 to 1.9 million people (Turner et al., 1989; Office of Technology Assessment, 1990; Research Triangle Institute, 1989; Valdiserri et al., 1993), and more than 3.2 million people have injected drugs at some point in their lives (National Institute on Drug Abuse, 1991). There is of course substantial variation across the country in the number of injectors within specific geographic areas. For example, typically high rates of injection drug use are observed in the Northeast, Miami, and Puerto Rico; moderate rates in the West; and low rates in the Midwest.

The experiences of several large urban areas have shown how infection among injection drug users can explode rapidly after the introduction of the virus into that population. For example, within two years the HIV prevalence1 rates in Edinburgh, Scotland, soared from 5 percent in 1983 to 57 percent in 1985 (Robertson et al., 1986); the rates in Bangkok, Thailand, increased from 1 to 43 percent in one year (Berkelman et al., 1989); and by 1985, seroprevalence estimate rates as high as 69 percent were reported in Milan, Italy (Titti et al., 1987; Angarano et al., 1985). Similarly in the United States, the virus spread rapidly in New York City in the early 1980s, with seroprevalence stabilizing at between 55 and 60 percent in 1984 through 1987 (Des Jarlais et al., 1989). The main factor associated with the accelerated increase in HIV infection among injection drug users is the widespread sharing of needles or syringes, which is in part a consequence of the restricted availability of sterile injection equipment. Moreover, it is a matter of concern that epidemiologic data indicate that, given the large number of injection drug users and their comparatively high incidence of HIV, the basic elements necessary for the rapid diffusion of the virus are apparent. That is, communities of injection drug users with high levels of both HIV infection and risk behaviors (i.e., involving drugs and sex) can serve as a bridge across distinct populations and efficiently impact the infection rate of other groups for which the HIV prevalence rates are currently relatively low. The epidemiologic data do indicate that the HIV epidemic in this country is now clearly driven by infections occurring in the population of injection drug users, their sexual partners, and their offspring.

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