In Randy Shilts’ history of AIDS, “And the Band Played On,” he tells the story of an Air Canada steward named Gaëtan Dugas, who suffered from what Dugas called “gay cancer” and infected 40 people or more with HIV.

He was, Shilts wrote, “Patient Zero.”

Dugas, through his extensive travels and unrepentant, unprotected sex even after he was diagnosed, undoubtedly helped spread AIDS. But was he the man who brought the disease to America?

In the new book “The Chimp and the River: How AIDS Emerged from an African Rain Forest” (W.W. Norton), author David Quammen says no.

“Dugas himself was infected by some other human, presumably during a sexual encounter — and not in Africa . . . somewhere closer to home,” Quammen writes. “As evidence now shows, HIV had already arrived in North America when Gaëtan ­Dugas was a virginal adolescent.”

Using molecular genetics, researchers have now traced the ­exact strain of HIV that became a pandemic — HIV-1, Group M, Subtype B — to its original source.

Amazingly, through examination of genetic samples from humans and chimps, Quammen reveals scientists have found exactly when and where AIDS started — even a probable theory as to how.

1908: Research reveals “AIDS began with a spillover from one chimp to one human, in or near a small southeastern wedge of Cameroon, around 1908,” Quammen writes. The most likely way it jumped species was through a person Quammen calls the “Cut Hunter” — a man who hunted and butchered a chimpanzee infected with simian immunodeficiency virus and was wounded in the process. The chimp’s blood mingled with his through the cuts in his skin.

1910s-20s: Chances are the Cut Hunter infected only one other person, and HIV likely spread on a one-to-one basis through sexual contact, working its way down the Sangha River in Cameroon to the Congo, eventually reaching the city of Leopoldville (later Kinshasa). Why did no one notice? Life expectancy in that time and place wasn’t that high. And the infected were likely to die of some other common disease, with no one suspecting that their immune systems were compromised.

1920s-50s: Colonial officials were conducting massive health campaigns in Africa to treat tropical diseases. Quammen notes that the treatment for one ailment, caused by tsetse flies, required 36 injections over three years. But hypodermic syringes were rare commodities, made out of glass and metal. They were used over and over again. “Once the reusable needles and syringes had put the virus into enough people — say, several hundred — it wouldn’t come to a dead end, it wouldn’t burn out, and sexual transmission could do the rest,” Quammen writes. Meanwhile, Kinshasa’s population exploded. Between 1940 and 1960, the city grew from 49,000 people to about 400,000.

1960: Belgium abruptly gives up the Congo as a colony as forces led by Mobutu Sese Seko overthrow the government. The Belgian regime, Quammen notes, had discouraged education among its colonial subjects; there were no Congolese doctors. Instead, those ranks were filled by French-speaking Haitians who fled home after the government fell. “Someone brought back to Haiti, along with Congolese memories, a dose of HIV-1, Group M, Subtype B,” Quammen writes.

1969: But how does one infected Haitian lead to an outbreak that, according to 1982 blood tests, results in 7.8 percent of women in a Port-au-Prince slum having HIV? Again, needles. In the early 1970s, a plasma-donation clinic, run by a Miami investor, opened in Haiti offering residents $3 per liter. Shared needles at this clinic likely increased the infection rates in Haiti and shipped the disease to the United States in frozen blood plasma. Research indicates that just a single migration of the virus — ­either one infected person or one container of plasma — accounted for bringing AIDS to America. “That sorry advent had occurred in 1969, plus or minus about three years,” Quammen writes.

1980: The disease lurked in America for a decade before anyone noticed. “It reached hemophiliacs through the blood supply,” Quammen writes. “It reached drug addicts through shared needles. It reached gay men … by sexual transmission, possibly from an initial contact between two males, an American and a Haitian.” In 1980, Michael Gottlieb, an assistant professor at UCLA Medical Center, noticed a number of gay men suffering from pneumonia because of weakened immune systems. He wrote a short piece about it in 1981 for the Centers for Disease Control newsletter. A similar cluster is documented in New York. At the same time, a group of heterosexual Haitian immigrants in Miami are found to suffer from symptoms similar to Gottlieb’s patients.

They are the first warnings of what is happening.