to injection drug use. Figure 1.1 illustrates the variation in AIDS incidence rates across states. Moreover, a closer look at selected metropolitan area AIDS cases reported in 1993 reveals some substantial variations within and across regions (Table 1.1).

In contrast to surveillance data on AIDS, precise estimates of HIV infection rates in the total U.S. population remain problematic. National estimates rely on mathematical models that back calculate HIV incidence from AIDS surveillance data as well as a composite of HIV seroprevalence data from numerous sources: states' reporting and screening of newborns, blood donors, armed forces recruits, Job Corps participants, persons attending alternative testing sites and sexually transmitted disease clinics, admissions to drug abuse treatment centers and prisons, and various other sentinel populations.3 Over the years, the Public Health Service (PHS) has estimated that there are between 600,000 and 1.2 million HIV-infected people in the United States and that approximately 40,000 new infections occur each year among adults and adolescents (U.S. Public Health Service, 1986; Centers for Disease Control, 1987, 1990a; MacQuillan et al., 1993). A discussion of the large observed variations in PHS estimates over the years is presented in Vermund (1991).

Mode of Acquisition of Infection

Important trends in the mode of acquisition of the HIV infection can be discerned. In this country and throughout the world, the majority of HIV infections are sexually transmitted (Roper et al., 1993). In most of the world, over 75 percent of HIV infections are due to heterosexual behavior, approximately 15 percent to homosexual behavior, and a relatively small proportion to injection drug use. However, in the United States, men who have sex with men account for the largest number of reported AIDS cases (54 percent of all reported cases, that is, 193,652 cases as of December 31, 1993). Yet a review of the percentage of annual AIDS cases (Figure 1.2), classified according to CDC's exposure categories, reveals that the proportion of cases of men who have sex with men has decreased steadily over the years (from 74 percent in 1981 to 47 percent in 1993), while the proportion of cases of exposure from injection drug use has steadily increased over the last 13 years (from 12 percent in 1981 to 28 percent in 1993).

In the United States, gay and bisexual men are still the largest risk group for HIV infection and disease. Drug users constitute the next-largest risk group, although there is a large overlap between these groups. Recent CDC estimates of HIV prevalence and incidence (Holmberg, 1993, 1994) indicate that the HIV/AIDS epidemic in the United States is being driven by three subepidemics: (1) injection drug users and their sexual partners and offspring (especially in the northeastern United States; Miami, Florida; and



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