This is a pretty interesting read. Jacques Pepin has been tracing the disease for decades, and says it got started in Cameroon, Africa when a hungry soldier killed a chimpanzee for food and was infected. It was then transmitted through reused needles to the sex worker population where it got spread all over. This is at the National Post:

The locals lived off cassava, other crops and bushmeat, but the soldiers were reliant on supplies sent from Léopoldville and Brazzaville that were sent up the River Congo and then carried up to 25 miles by foot by locals, the Daily Mail reported. It was hard to get enough supplies and many soldiers starved to death, forcing others to hunt for food.

“All of a sudden you have 1,600 soldiers with rifles and plenty of ammunition, so the level of hunting in that area went up dramatically over these few months” in 1916, he told the Daily Mail.

“My hypothesis is that one of the soldiers got infected while hunting in the forest. A chimpanzee was killed and when cutting the animal to bring it back, there was an injury which got infected with the virus. Eventually, the soldier, after the war, came back all the way to Léopoldville and probably started the very first train of transmission in Léopoldville itself.”

The researcher has written a book called The Origin of Aids.