The AIDS virus has been circulating among people for about 100 years, decades longer than scientists had thought, a new study suggests.

Genetic analysis pushes the estimated origin of HIV back to between 1884 and 1924, with a more focused estimate at 1908. Previously, scientists had estimated the origin at around 1930. AIDS wasn’t recognized formally until 1981 when it got the attention of public health officials in the United States.

The new result, which appears in the latest issue of the journal Nature, is “not a monumental shift, but it means the virus was circulating under our radar even longer than we knew,” says Michael Worobey of the University of Arizona, an author of the work.

Scientists say HIV descended from a chimpanzee virus that jumped to humans in Africa, probably when people butchered chimps. Many individuals were probably infected that way, but so few other people caught the virus that it failed to get a lasting foothold.

But the growth of African cities may have changed that by putting lots of people close together and promoting prostitution, Worobey suggested. “Cities are kind of ideal for a virus like HIV,” providing more chances for infected people to pass the virus to others.

Experts say it’s no surprise that HIV circulated in humans for about 70 years before being recognized. An infection usually takes years to produce obvious symptoms, a lag that can mask the role of the virus, and it would have infected relatively few Africans early in its spread.

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