AIDS was present in chimps a long time and occasionally crossed to humans in the bush, probably at some constant low rate over centuries. It never went far due to low life expectancy and small communities.
Once the Belgians went in and reused lots of glass syringes doing colonial do-gooding vaccination for sleeping sickness, it spread around a whole lot for the first time and jumped from the bush to the emerging large cities.
The migration to the cities was largely young males looking for work and hence once it got into the emergent city sexworker community it spread like crazy.
The Haitians sent francophone medical staff to that region as a helping hand and they spread it back to Haiti.
It also jumped out in various other ways.
Once in NYC, the partner count and high rates of transmissivity through anal sex spread it though gay males. There was also a famous gay and promiscuous flight attendant.
Once into the blood supply it hit hemophiliacs at high rates (relative to other groups).
This is the overview of how the 3 Hs initial focus occurred. Significant bigotry and political fuckery got it ignored for a while until actual research could figure out it was a disease everybody could get and that we should fucking well use science and medicine rather than demonizing people with HIV/AIDS.
Oh wait, we still do that.
For further reading, try Pepin.