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Following its recognition in 1981, the HIV/AIDS epidemic has evolved to become the greatest challenge in global health, with some 34 million persons living with HIV worldwide. Early epidemiologic studies identified the major transmission routes of the virus before it was discovered, and enabled the implementation of prevention strategies. Although the first identified cases were in MSM in the United States and western Europe, the greatest impact of the epidemic has been in sub-Saharan Africa, where most of the transmission occurs between heterosexuals. Nine countries in southern Africa account for less than 2% of the world's population but now they represent about one third of global HIV infections. Where broadly implemented, HIV screening of donated blood and antiretroviral treatment (ART) of pregnant women have been highly effective in preventing transfusion-associated and perinatally acquired HIV, respectively. Access to sterile equipment has also been a successful intervention for injection drug users. Prevention of sexual transmission has been more difficult. Perhaps the greatest challenge in terms of prevention has been in the global community of MSM in which HIV remains endemic at high prevalence. The most promising interventions are male circumcision for prevention of female-to-male transmission and use of ART to reduce infectiousness, but the extent to which these interventions can be brought to scale will determine their population-level impact.