This groundbreaking work, with its unique anthropological approach, sheds new light on a central conundrum surrounding AIDS in Africa and in so doing, reframes current debates about the disease. Robert J. Thornton explores why HIV prevalence fell during the 1990s in Uganda despite that country's having one of Africa's highest fertility rates, while, during the same period, HIV prevalence rose in South Africa, a country with Africa's lowest fertility rate. Using anthropological, epidemiological, and mathematical methods, Thornton finds that culturally and socially determined differences in the structure of sexual networks - rather than changes in individual behavior - were responsible for these radical differences in HIV prevalence. His study exposes these invisible networks, or unimagined communities, unseen both by those who participate in them and by the social sciences, and opens a new area of investigation - the sexual network as social structure. Incorporating such factors as property, mobility, social status, and political authority into our understanding of AIDS transmission, Thornton offers a fresh vision of the disease, one that suggests new avenues for fighting it worldwide.