Global killer ... scientists have pinpointed the origin of the AIDS virus from chimp to human in Cameroon. Photo: Reuters

Following a trail of death, Craig Timberg and Daniel Halperin tell the grim story of the birth and spread of AIDS.

WE are unlikely to ever know all the details of the birth of the AIDS epidemic. But a series of recent genetic discoveries have shed new light on it, starting with the moment when a connection from chimp to human changed the course of history.

We now know where the epidemic began: A small patch of dense forest in south-eastern 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. We typically think of diseases in terms of how they threaten us personally. But they have their own stories. Diseases are born. They grow. They falter and sometimes they die. In every case, these changes happen for reasons.

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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 travelled only sporadically by humans before.

The most disruptive of these intruders were thousands of African porters. Forced into service by European colonial powers, they cut paths through the exact area that researchers have now identified as the birthplace of the AIDS epidemic. It was here, in a single moment of transmission from chimp to human, that a strain of virus called HIV-1 group M first appeared.

In the century since, it has been responsible for 99 per cent of all of the world's deaths from AIDS not just in Africa but in Moscow, Bangkok, Rio de Janeiro, San Francisco, New York and Washington. All that began when the West forced its will on an unfamiliar land, causing the essential ingredients of the AIDS epidemic to combine.

It was here, by accident but with motives by no means pure, that the world built a tinderbox and tossed in a spark.

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Anatomy of a plague