Maybe it’s too much to say that imperialism caused AIDS. But at the very least, it’s karma—payback, if you will, for mass slaughter, slavery and vicious exploitation in central Africa more than a century ago.

That’s the point of a stunning excerpt of a new book in the Washington Post today. The book is Tinderbox: How the West Sparked the AIDS Epidemic and How the World Can Finally Overcome It, by Craig Timberg and Daniel Halperin, and it’s high on my reading list now. Its point is that the penetration of Africa by Belgians, Germans and other colonialists in the late nineteenth century not only brought syphilis to Africa, a disease that killed countless Africans, but unleashed the AIDS virus, too. From “a small patch of dense forest in southeastern Cameroon,” the HIV-1 Group M virus catapulted itself from monkey to man, traveling down a river system to the colonial outpost of Kinshasa sometime between 1880 and 1910 or so.

Some of the research that informs this book has been circulating for a while. Much of it was conducted by Beatrice Hahn of the University of Alabama at Birmingham and Paul Sharp of the University of Edinburgh and published in Science in 2006. But Tinderbox puts it all together in a way that shows the consequences of imperialism, colonialism and the attendant public health disaster that followed it.

We know it now, thanks to geneticists, evolutionary biologists and other scientists that Republicans and Christian evangelicals apparently don’t believe in.

As the excerpt tells us:

For decades nobody knew the reasons behind the birth of the AIDS epidemic. But it is now clear that the epidemic’s birth and crucial early growth happened during Africa’s colonial era, amid massive intrusion of new people and technology into a land where ancient ways still prevailed. European powers engaged in a feverish race for wealth and glory blazed routes up muddy rivers and into dense forests that had been traveled only sporadically by humans before.

Careful scientific detective work, tracing the HIV virus from 1950s and 1960s-era blood and tissue samples back through time, partly by studying thousands of samples of monkey feces across central Africa, allowed the story to unfold.

The excerpt includes this gripping passage (but read the whole thing, and then buy the book):

So HIV’s first journey looked something like this: A hunter killed an infected chimp in the southeastern Cameroonian forest, and a simian virus entered his body through a cut during the butchering, mutating into HIV.

This probably had happened many times before, during the centuries when the region had little contact with the outside world. But now thousands of porters—both men and women—were crossing through the area regularly, creating more opportunities for the virus to travel onward to a riverside trading station such as Moloundou.

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.