NEW YORK (AP) -- Researchers have pushed back the origin of the AIDS virus to as early as 1884.

A report in tomorrow's issue of the journal Nature says genetic analysis estimates that HIV arose between 1884 and 1924, with a more focused estimate at 1908. The previous estimation was around 1930.

Scientists say the new time frame corresponds with the growth of cities in Africa, where the virus jumped from chimpanzees to humans. Researchers say cities put lots of people close together, and promote prostitution.

Experts say it's no surprise HIV circulated in humans for about 70 years before being recognized. They say an infection usually takes years to produce obvious symptoms, and it would have infected relatively few Africans early in its spread.

AIDS wasn't recognized formally until 1981.

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