The main cause could be lack of resources that can protect people from the diseases that sex can bring, basically unprotected sex ( could be with multiple partners making the likelihood of aids even greater). Also in Africa because of extreme areas of poverty, uneducated, and down trodden areas many people dont have proper hygiene which can also increase chances of aids ( of course when sex is involved). Also remember that aids can occur from more than just sex alone, it is the result of contact with contaminated blood.
