I am going from memory here, and the research changes over the years.
HIV has jumped to humans at least twice. Once directly from a monkey. The second time from monkey to chimp to human. The route is probably butchering/eating raw meat. So we have two major strains of HIV. Of one of these strains, there are about 6 major varieties. We are familiar with the more successful jumps, but there are very limited successes. One strain has under a dozen infected in Africa. And this difficulty spreading in a new host species is not uncommon. Measles appears to have taken hundreds of years to get so well adapted to humans (500 AD to 1200 AD according to last source I read).
Probably sometime a little before WW1, HIV jumped to humans in either what was Kamerun or the French Congo. The areas it jump are absolutely remote. They are malaria filled. And there was major issues with West African Sleeping sickness epidemic in that time frame. Aids is spreading, but not in an area that Westerners are likely to document or care about. In the 1920's, Kinshasa was likely the first major urban area for Aids to get established. It probably traveled to the city via truckers and brothels. Aids thrived in this environment, but no one noticed. The Belgians did not want to educate the local but need non-European high skill workers such as Doctors. The Belgians imported Haitians. When the Congo became free, these Haitians returned to Haiti and established the disease there. Again, no one notice despite a major vector Haiti being medical doctors. Aids then spread to the USA via Haiti, probably just in workers going back and forth. Since it appears to be deeply established in Haitian communities in the USA, it probably was not primarily moved by the sex vacation route. No one picked it up in ethnic communities in the USA. Years later, then we get to "Patient Zero" who is only important because of the political decisions made based on this information. He was a airline employee who was gay. He probably got the disease in Africa, but my memory is a bit vague on this one. He then infected three or four people. About 6 years later, they all come down with the same illnesses in short order and the same doctor treats them. We are lucky that these guys did not move to different cities in the half decade or more from infection to sickness. We are also lucky all three of these guys had sex with the same man about 3-6 months before getting sick. And that they fell ill in the order they had sex with this man. Originially, the doctors thought AIDS had a incubation period of months, and the evidence used to determine the cause was false. Then later they figure out that "slims" in Africa was AIDS
So not only did we miss AIDS many times, we got lucky to catch it when we did in the USA. I don't think anyone looked for infectious diseases where there was a 7 year infectious period with no symptoms. Blinded by our knowledge of diseases, so to speak. So a lot of words to get to.
We could have easily missed a more infectious AIDS or an earlier jump of AIDS to humans. Personally, I suspect that AIDS jump to humans many times well before 1900. Eating monkeys is not uncommon, and if you have butcher wild animals, you will know how much blood is involved and how you can easily have small cuts on your hands. And I suspect that when it became common enough, the more deadly infectious diseases wiped out the small communities with AIDS. By small, I mean a few hundred here or there.
Europeans caused a big spike in disease with railroads and forced labor conditions that resemble slavery. In some pretty large areas, it looks like populations declined as Europeans did the typical colonial stuff. To give you an example, the Serengeti went into its wild state with the last 100 years. People used to live there until Eastern African Sleeping sickness became stronger. Besides enriching a very few whites, African colonization is a testament to inept governance.