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Over the past decade, limited attention has been paid to the human immunodeficiency virus (HIV) epidemic in the United States. The global epidemic — particularly the epidemic in sub-Saharan Africa, where approximately two thirds of the world's population living with AIDS resides — has rightfully received most of the focus. Meanwhile, however, the prevalence of HIV infection within some U.S. populations now rivals that in some sub-Saharan African countries (see graph). For example, more than 1 in 30 adults in Washington, D.C., are HIV-infected — a prevalence higher than that reported in Ethiopia, Nigeria, or Rwanda.1 Certain U.S. subpopulations are . . .