This is a good point that's been brought up before. If a guy catches HIV in rural Africa during the 1700's who is he going to pass it to? His wife/wives. That's it. For all we know this crossover event could have happened dozens of times but since there wasn't anyone to pass it on to, the virus stayed local.
Late 1800's/early 1900's come around and you have colonization, better trade routes, and larger hubs. And what's a good trading hub without a hooker or two? That traffic is where you get the spread from.
his wife/wives would have passed it down to the children, as well homosexuality has always been around it would have been passed.