For tens of thousands of years, some types of monkeys in West Africa have lived with a virus that attacks the immune system. Two different strains of it developed, one in Cameroon and one in the Senegal-Ivory Coast region. It wasn’t fatal because it was a “weak” virus. The immune system counter-attacked and fended off the virus. Over-simply put, it couldn’t evolve because the immune system jumped on it before the virus could make genetic changes. For the virus to evolve fast enough to become dangerous, it would have to be passed rapidly from one host to another—making changes along the way–before the immune-system hammered it flat.
So, no big deal even for diseased monkeys. However, American Indians hunted buffalo for food and Africans hunted “bushmeat” for food. Monkeys, tapirs, stuff like that. Then you had to skin them out to make steaks, chops, Kentucky Fried Tapir, etc. Butchers got nicked by knives and covered in (infected) blood along the way. So, the monkey immune-virus got into humans through open cuts. Still, this was not a big deal for most people because most Africans lived in isolated and not-very-large communities. That is, they lived in villages in the middle of forest clearings. The human version of the monkey immune deficiency, HIV, didn’t transmit very well because it needs to enter the bloodstream in some fashion. Even normal sex won’t do it most of the time. So, it settled into humans like it had settled into monkeys, and the human immune system fell on it.
So, no big deal even for diseased humans. However, European imperialists took over West and Central Africa in the late 19th Century. They tried to turn the place into a paying concern by starting plantations, building ports and railroads and warehouses, and corralling a lot of African labor. Port cities, railroad towns, and mining camps sprang up. All had large African working populations and small European ruling populations. People kept pouring in from the countryside. Most were men, but a minority were women.
The truth of the matter is that guys will pay for sex. Prostitution thrived in the towns of Central Africa. So did venereal diseases like syphilis. These diseases cause genital ulcers. Such ulcers greatly facilitated the rapid spread of HIV into the human bloodstream. Rapid transmission allowed rapid genetic evolution into the fearsome plague we know today. Widespread vaccination of workers against smallpox compounded the problem because the colonial medical authorities did not sterilize needles between injections in order to save money.
So, a big deal for African workers, but no big deal for anyone else. However, in the mid-1960s, HIV reached the Western Hemisphere and Europe. In all likelihood, a merchant seaman carried it from one port to another. In the early 1960s a young Norwegian merchant sailor spent time in West Africa. He came down with gonorrhea and was infected with HIV. By 1968 he had abandoned the sea to work as a long-haul truck-driver in Europe. He often had sex with prostitutes while on trips. He died in 1976. In 1966 a boy in St. Louis contracted HIV by some unknown means and died of it in 1969. Was a closeted gay man working in tropical medicine at one of the St. Louis universities? HIV/AIDS was still unknown outside Africa at the time and American and European doctors were stumped. About the same time some still-unidentified person traveled from the newly-independent Democratic Republic of the Congo to Haiti. S/he carried HIV. An epidemic soon began in Haiti. From here it was communicated in much greater numbers to the United States. By 1981 doctors had begun to identify an HIV/AIDS epidemic.
 See: US Army training films during the Second World War.
 I’m not making this up. Alas.