The Washington Post has a fascinating excerpt from Craig Timberg and Daniel Halperin's forthcoming book Tinderbox. Money quote:
One of the first victims — whether a hunter, a porter or an ivory collector — gave HIV to a sexual partner. There may have been a small outbreak around the trading station before the virus found its way aboard a steamship headed down the Sangha River.
For this fateful journey south, HIV could have ridden in the body of these first victims, or it could have been somebody infected later: a soldier or a laborer. Or it could have been carried by a woman: a concubine, a trader.It’s also possible that the virus moved down the river in a series of steps, maybe from Moloundou to Ouesso, then onward to Bolobo on the Congo River itself.
There might even have been a series of infections at trading towns along the entire route downriver. Yet even within these riverside trading posts HIV would have struggled to create anything more than a short-lived, localized outbreak.
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. People can have sex hundreds of times without passing the virus on. To spread widely, HIV requires a population large enough to sustain an outbreak and a sexual culture in which people often have more than one partner, creating networks of interaction that propel the virus onward.
To fulfill its grim destiny, HIV needed a kind of place never before seen in Central Africa but one that now was rising in the heart of the region: a big, thriving, hectic place jammed with people and energy, where old rules were cast aside amid the tumult of new commerce.
It needed Kinshasa. It was here, hundreds of miles downriver from Cameroon, that HIV began to grow beyond a mere outbreak. It was here that AIDS grew into an epidemic.