rate of partner change is simplistic. Instead, it appears that the simultaneous occurrence of several risk factors for HIV transmission determines how rapidly and to what level HIV spreads among the population and who will become infected. In the absence of facilitating factors, HIV infection could remain endemic at low levels over long periods of time until a critical prevalence of infection is reached, and the spread of HIV-1 accelerates (Nzilambi et al., 1988). This epidemiologic diversity not only reflects differences in sexual and other behaviors, but also suggests that the epidemic has not reached an equilibrium in most areas.
The epidemiological evidence suggests that HIV prevalence may be stabilizing in some large urban centers (see Figure 3-4) and potentially in some rural areas (Wawer et al., 1994b; U.S. Bureau of the Census, 1994a). However, it must be recognized that a stable prevalence can conceal a significant level of new HIV infection replacing those who die (Wawer et al., 1994b). For example, in the absence of migration, a stable adult seroprevalence of 20 percent suggests that up to 2 percent of adults become newly infected each year, replacing the approximately 10 percent of the infected who are expected to die annually in African settings.
The HIV epidemic and the demographic structure of sub-Saharan populations will have complex interactions over time. The population of sub-Saharan Africa is predominantly young, in sharp contrast with the age structure in developed countries; 45 percent of the population of the region is under the age of 15, compared with one-third or less for the other major geographic regions (Decosas and Pedneault, 1992; Quinn, 1994). Among persons aged 15 and over, those in the 15-39 age group represent over two-thirds of all sub-Saharan adults; only in Latin America do young adults so predominate, whereas in Asia and the developed regions, young adults represent at most half of all adults (United Nations, 1993). In urban areas, one finds a prominent one-sided bulge caused by the migration of young males into the cities for employment (with some rural areas reporting a proportional ''deficit" of young males who have migrated away) (Serwadda et al., 1992). For example, the prevalence of HIV infection among both urban and rural populations in Uganda is highest in the 25-to-44-year-old bracket among males and in the 15-to-34-year-old bracket among females (Figure 3-5) (Wagner et al., 1993; Serwadda et al., 1992).
A significant contributor to the elevated prevalence of infection in sub-Saharan Africa is the fact that behavioral factors associated with HIV transmission—including multiple partners and impermanent relationships—are generally more common among the young, this coupled with the high proportion of young adults found in sub-Saharan African countries (Anderson et al., 1991). Accordingly, the large number of young persons under age 15, who will soon enter their