We now know where the epidemic began: a small patch of dense forest in southeastern Cameroon. We know when: within a couple of decades on either side of 1900. We have a good idea of how: A hunter caught an infected chimpanzee for food, allowing the virus to pass from the chimp’s blood into the hunter’s body, probably through a cut during butchering.
As to the why, here is where the story gets even more fascinating, and terrible.
Timberg and Helperin explain that the hunter was infected around 1900, during Africa's colonial era--a propitious moment for transmission, as thousands of people were trundling through previously isolated areas of West Africa trying to open and secure trade lines. From Cameroon, they explain, the strain in question made its way to Kinshasa, where it exploded:
Most of this colonial world didn’t have enough potential victims for such a fragile virus to start a major epidemic. HIV is harder to transmit than many other infections...
So the improbable journey of the killer strain of HIV was feasible for only a few hectic decades, from the 1880s to the 1920s. Without “The Scramble for Africa,” it’s hard to see how HIV could have made it out of southeastern Cameroon to eventually kill tens of millions of people. Even a delay might have caused the killer strain of HIV to die a lonely death deep in the forest.
But as it happened, "the West forced its will on an unfamiliar land, causing the essential ingredients of the AIDS epidemic to combine." This is fascinating as history and as a scientific detective story. I'm not sure what to make of the implied comment on colonialism. As the authors explain, it's tragic that this crucial early infection happened at time of expanded interaction; if not for that, the HIV strain in question might have wreaked its havoc on a handful of people rather than tens of millions around the world.
But we know that interactions between previously separate groups can have catastrophic consequences as well as productive ones. Spain devastated the Aztecs with smallpox, for example, and the Mongol armies brought gunpowder to the west. It's no defense of colonialism to say that with regard to transmission (of disease, technology, or ideas) the beh matters more than the motive. That seems like a salient point here, because while people now reject colonialism, some of the actions thereof--global trade and travel--obviously have only increased. For more on the relationship between economic interchange and the rate of HIV infection, see this TED talk from economist Emily Oster--more exports mean more AIDS, as she puts it (starting around minute 10).