Mostly cultural factors.
In many parts of the godforsaken continent, it is common for men to have many wives and wives to have many husbands. I would also imagine that the lack of a good diet and hygiene would also effect the immune system.
Maybe the UN should begin preaching to the Africans (as opposed to the U.S.) on the need for sustainable development. Don't have ten kids if you live in a desert and can barely feed yourself kind of advice. (Sam Kinison did a great commentary on this)