I believe as I have heard other scientists report that the reason for alot of blacks and other races being so succeptible to the actual AIDS is due to a genetic immunity from the Black Death in Europe.
They are now thinking that the Black Death was not the bubonic plague because the bubonic plague is a bacteria carried by rats and Iceland was hit with this plague where there were no rats or mice. So they believe the Black Death was viral in nature and a variation of the HIV virus, but more virulent.
This would make sense why there are so many cases of full blown AIDS in Africa and why they seem to die so much quicker.
The practical reason is that there is a group of people that engages in certain behaviors mostly with other people in that group.