I think sickle cell anemia persists because it also gives its victims a partial immunity to malaria. So they are more likely to survive Malaria and less likely to survive the anemia and these two end up in equilibrium in Africa.

There still does seem to be some question as to whether Africans are more susceptible to HIV than other races. The percentages of the population with HIV in some African countries is so high it seems unlikely this is purely caused by environmental factors like nuitrition, santiation, promiscuity, etc.

There are plenty of other places in the world with some or all of these where AIDS has not spread as much.

One theory is that AIDS was given a head start by medical intervention, such as polio vaccinations or other mass inoculations with dirty needles. If so then it may have reached a sufficient level in the population that overcame its natural difficulties in transmission.

It is believed that smaller outbreaks of AIDS occured in the past as eating monkeys is a traditional food in much of Africa. It no doubt would have been common for people cutting up monkey meat to have cut themselves and allowed monkey blood to get in the cuts. Also sores in the mouth can be another factor if eating raw monkey meat, mainly Chimpanzee.

If someone was infected it would have been likely that they would have either died or became sick enough to be ostracised before they could pass it on. Sex is a poor way of transmission and homosexuality is rare enough that a gay community would not be likely to be found of sufficient size. So an AIDS infection would likely die out.

Another factor is that when a disease like AIDS crosses into another host like humans it is usually more virulent and people die much faster, as we saw in the initial stages of the AIDS epidemic.

Through natural selection to some degree (and perhaps other factors) the pathogen becomes less virulent and so survives longer to infect others. Usually a disease does not kill its host as this is an inefficient way for it to survive.

Even now in Western countries AIDS is relatively well contained by behaviour changes which also indicates it has not adapted to the difficulties in transmitting itself. That the situation is so different in Africa may indicate the strains there are still very virulent. It would be interesting to see if African Americans are more susceptible, IIRC they are even in the US.

If the disease spreads rapidly enough then the more virulent strains are not weeded out as they are also passed on quickly. So it is possible that AIDS itself is much more dangerous in Africa because of this.

I read an interesting book on AIDS called "The River" in which it was suggested polio vaccinations and trials in the Congo in the 50's may have led to the start of AIDS. While there are no samples of vaccine surviving that indicate this, there is apparently no case of AIDS existing before 1957.

So the question arises, why didn't AIDS break out of its small pockets of infections before 1957?