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Most books about AIDS start in 1981, when gay American men began dying of a rare pneumonia. In “The Origins of AIDS,” published last week by Cambridge University Press, Dr. Jacques Pépin, an infectious disease specialist at the Université de Sherbrooke in Canada, performs a remarkable feat.

Dr. Pépin sifts the blizzard of scientific papers written about AIDS, adds his own training in epidemiology, his own observations from treating patients in a bush hospital, his studies of the blood of elderly Africans, and years of digging in the archives of the European colonial powers, and works out the most most likely path the virus took in the course of the years it left nearly no tracks.

Operating slowly forward from 1900, he explains how Belgian and French colonial policies led to an incredibly unlikely event: a fragile virus infecting a modest minority of chimpanzees slipped into the blood of a handful of hunters, 1 of whom must have sent it down a chain of “amplifiers” — disease eradication campaigns, red-light districts, a Haitian plasma center and gay sex tourism. With out those amplifiers, the virus would not be what it now is: a grim pilgrim atop a mountain of 62 million victims, living and dead.

In the early 1980s, Dr. Pépin was a young physician fighting a sleeping sickness epidemic at a hospital in Nioki, in what was formerly the Belgian Congo, then Zaire, and is now the Democratic Republic of Congo. The virus was then unknown in Africa, but his work gave him clues that would later support him on its trail.

In retrospect, he says in his book, he may possibly have inadvertently infected some of his patients. Ideally, the glass syringes utilized in Nioki were sterilized in the hospital’s autoclave. But with the electricity frequently out, nurses boiled them instead. “And I did not pay too much attention to how long they had been boiled,” he said in an interview.

Later, he worked in Guinea-Bissau on H.I.V.-two, which is associated to H.I.V.-1 but causes a milder and tougher-to-transmit form of AIDS that some victims live with for decades. Noting that situations had been much more common amongst older individuals, he concluded that it was dying out. If sexual transmission amongst young folks was not keeping it alive, he reasoned, some other route should have first created it so widespread among the elderly. He suspected the aggressive campaigns that colonial physicians waged against syphilis, yaws, leprosy, tuberculosis and other ills till independence arrived in the 1960s. They all utilized injections, considering that pill versions of numerous antibiotics did not exist or were pricey.

In 2005, Dr. Pépin began field studies. By sampling the blood of Africans 55 and over, he showed that those who had several injections in their youth or had undergone ritual circumcision, in which a lot of boys had been cut with the very same blade, usually had antibodies to hepatitis C or HTLV, a little-recognized virus that, like H.I.V.-1, comes from chimps and infects the CD4 cells of the immune program, but is harmless.

That was challenging evidence that blood and syringes had spread other viruses.

Blood and tissue samples stored in freezers in Africa and in European hospitals that treat Africans — a couple of going back to the 1950s — form a map of AIDS viral subtypes, which is surprisingly complex. For example, white and black South Africans have distinct subtypes. “Few homosexual Afrikaners have sex with heterosexual Zulus,” Dr. Pépin notes. The whites’ subtype is typical amongst gay European and American males the 1 most widespread amongst blacks moved south by means of Zambia.

Simian immunodeficiency virus, which infects monkeys and apes, is similarly mapped it was very first located in zoo animals, but now is tracked by jungle teams who extract DNA from feces.

The ancestor to AIDS is in 1 chimpanzee subspecies, Pan troglodytes, which in nature lives only among the Sanaga and Congo Rivers. (Chimpanzees can’t swim.) It is a blend of simian viruses from red-capped mangabeys and mustached guenons, small monkeys that chimps hunt and eat.

From colonial archives in Paris, Marseilles, Brussels, Lisbon and London, Dr. Pépin dug out old records of clinics where, as early as 1909, African prostitutes were required to have venereal illness inspections. He went by means of stacks of newspapers, like the Voix du Congolais, which wrote extensively about polygamy and prostitution, and pored over studies by European ethnographers.

(His own fluency in French was crucial, of course.)

In brief, his recounting of the epic journey is this:

In nature, only about 6 percent of troglodytes chimps are ever infected. Within a troop, every female mates with many males, but mating with outsiders is rare, so most troops are untouched while a couple of are heavily infected.

H.I.V.-1’s 4 genetic groups, M, N, O and P, show that it produced the chimp-human jump at least 4 occasions in history. But group M accounts for much more than 99 percent of all circumstances.

Why did only 1 spread?