Explainer: Where and when did HIV begin?

In 1981, two research groups raised an alarm about a mysterious new infection. They reported that it was killing gay men in California and New York. The more scientists looked, however, the more they found signs that the disease might not be so new after all. In fact, it may have evolved almost a century earlier and in another part of the world.

By testing blood and tissue samples that had been stored in labs for years, researchers discovered that the virus had killed a doctor from Denmark in 1977. Then other researchers found that the virus had likely killed a sailor from Norway in 1976 and a teen from St. Louis, Mo., in 1969. Using the same methods, still other scientists found that HIV had infected a man in central Africa as early as 1959. He lived in what is now the Democratic Republic of the Congo.

In fact, some researchers now think the very first HIV infection in humans may have happened more than 100 years ago, in 1908 or earlier. In his new book, Spillover (2012, W.W. Norton & Co), author David Quammen describes how viruses that attack wild animals can sometimes “spill over.” That means the viruses can start attacking people.

Scientists found clues to suggest this happened with HIV after detecting a very similar virus in monkeys and in chimpanzees and other great apes. It’s called SIV, which stands for simian immunodeficiency virus. The types most similar to HIV appeared in chimpanzees living in the African country of Cameroon. Based on the most recent research, Quammen reports, SIV may have spilled over in that part of Africa and become the virus known as HIV. How? Researchers believe a chimp somehow passed the virus on to a person.

Some scientists suspect that a hunter killed an infected chimp for food (also known as bushmeat). In the process, the hunter could have come into contact with the animal’s infected blood. Perhaps the hunter had a cut or open sore. After jumping to people, Quammen writes, the virus then likely simmered for decades in nearby villages and slowly spread from one villager to another. If people died from other causes or from a second infection that overwhelmed their damaged immune systems, no one would have known they had HIV.

Eventually, a villager with no obvious signs of the infection might have traveled by river to a bigger town or city. In that new site, many more people could have picked up the virus and spread it widely. One of those likely spreading sites was a city now called Kinshasa, in the Democratic Republic of the Congo.

We may never know exactly how HIV was able to keep going at first. But after it spread for many years within western Africa, scientists suspect the disease jumped across the Atlantic Ocean when an infected person moved to Haiti. Once there, it finally bubbled over and became a global threat.