For a given unprotected sexual relationship with an HIV-infected person, Africans are between four and five times more likely than Americans to become infected with HIV themselves. This stark fact accounts for virtually all of the difference in population-wide HIV rates in the two regions.via Esquire
There is more than one reason why HIV spreads more easily in Africa than America, but the most important one seems to be related to the prevalence of other sexually transmitted infections. Estimates suggest that around 11 percent of individuals in Africa have untreated bacterial sexually transmitted infections at any given time and close to half have the herpes virus. Because many of these infections cause open sores on the genitals, transmission of the HIV virus is much more efficient.
So what do we learn from this? First, the fact that Africa is so heavily affected by HIV has very little to do with differences in sexual behavior and very much to do with differences in circumstances. Second, and perhaps more important, there is potential for significant reductions in HIV transmission in Africa through the treatment of other sexually transmitted diseases.
Such an approach would cost around $3.50 per year per life saved. Treating AIDS itself costs around $300 per year. There are reasons to provide AIDS treatment in Africa, but cost-effectiveness is not one of them.