been relatively stable at 5 to 6 percent since the mid-1980s (Piot et al., 1990; Piot and Tezzo, 1990).

Although the epidemic in Africa was first recognized in Central and East Africa, and these regions continue to have the highest infection levels, there is increasing evidence that the epidemic is spreading into West and Southern Africa. In Abidjan, Cô d'Ivoire, in West Africa, HIV-1 prevalence among adults increased from 1 percent in 1986 to more than 15 percent in 1992. In Nigeria, a country with more than 105 million inhabitants, the largest population in the region, studies indicate that the prevalence of HIV-1 and HIV-2 among prostitutes in Lagos may be rising rapidly (Dada et al., 1993; Olaleye et al., 1993; see also below).

Within the last few years, investigators have noted the introduction of HIV among high-risk populations in Nigeria. Although relatively rare during the late 1980s, HIV has been increasing since 1990 throughout Nigeria, according to several surveys. This trend is of critical importance since the population of Nigeria, estimated at over 105 million, represents more than one-sixth of the total population of sub-Saharan Africa (World Bank, 1995). In one recent study, 12.3 percent and 2.1 percent of 885 female prostitutes in Lagos State were infected with HIV-1 and HIV-2, respectively, a rise from a combined prevalence of only 1.7 percent 2 years previously (Dada et al., 1993). Women in the youngest age group, ages 12 to 19, had the highest prevalence (20 percent). In addition, prostitutes residing in the port area of Lagos, which serves as a major convergence of overland and sea routes within and outside Nigeria, had the highest prevalence of HIV-1 infection. A highway region that is traversed by the overland interstate highway also had high rates. Because Lagos is the largest cosmopolitan city in Africa, the constant migratory movement of people into and out of this major trade center provides further opportunity for HIV dissemination.

The virus may be spreading even more rapidly in Southern Africa than in West Africa. For example, in Botswana, HIV prevalence among pregnant women increased from 10 percent in 1991 to 34 percent in 1993 in Francistown, and from 6 percent in 1990 to 19 percent in 1993 in Gaborone (U.S. Bureau of the Census, 1994c). Similar disturbing data are emerging from South Africa, suggesting a three-fold increase in HIV prevalence between 1990 and 1993 among women attending prenatal clinics in most regions of the country. Aggregated data collected in prenatal clinics across South africa show a rapid increase in overall prevalence from under 1 percent in 1991 to 1.7 percent in 1992, 2.8 percent in 1993, and 6.4 percent in 1994 (U.S. Bureau of the Census, 1994a).

Thus, although HIV infection rates are high among many populations and subgroups in sub-Saharan Africa, there remains much variation in incidence and prevalence rates recorded to date, both geographically and by population subgroups. The probable causes of this heterogeneity in seroprevalence are multiple, and include behavioral, biological, and societal factors. Trying to explain the phenomenon by a single factor such as civil war, male circumcision, STDs, or

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