[HIV/AIDS] has an extremely high prevalence among heterosexual women in Sub-Saharan Africa; 40% of pregnant women in Botswana and 25% in South Africa are infected with the disease. . . .
[A new study by] Jeremy Magruder . . . at the University of California at Berkeley . . . shows that African women have become infected with HIV through doing what we all do; shopping for a mate. . . . Women and men in Africa experience a brief period in their lives while searching for the right marriage partner in which they are in a series of monogamous relationships with high turnover. This searching behavior generates a constant pool of individuals in short-term relationships. Rates of HIV transmission . . . from a person who has become recently infected is as much as 10 times higher. So, introduce one recently infected person into this pool of people who are searching for a mate and the whole pool are at risk of being infected. . . . the introduction of one person per hundred into a searching pool will lead to an HIV prevalence similar to that in Kenya or Tanzania. The introduction of just three infected people per hundred into the searching pool will lead to South Africa's epidemic prevalence rates. . . .
The economic model suggests that if only 50% of the participants in the marital searching pool use condoms for the first three months of their relationships, the prevalence rates would drop to those comparable to the West.
This is huge, although I suspect that it overstates the case somewhat because the mathematical model probably assumes greater homogeneity in relationship turnover rates than is present in reality, and it is safe to guess that a subgroup that is more likely to use condoms is also a subgroup that has a somewhat lower velocity of transmission. Still, the key observation, which is that herd immunity effects associated with condom use by a large share of the relatively small subgroup of people who are in the first three months of a sexual relationship, can dramatically reduce the proportion of people who get AIDS, is surely correct and is an attainable goal to achieve with targeted public relations and condom distribution campaigns.
The other practice that I've seen elsewhere noted as protective is male circumcision which reduced transmission risk by about 61% in a rigorous 2005 study. Nota bene: The benefits of circumcision for men who have sex with men is not so clear.