The strain of the HIV virus which predominates in the United States and Europe has been traced back to Haiti by an international team of scientists.
The virus strain passed from Haiti to the US in around 1969 before spreading around the globe, say the scientists.
The HIV virus can lead to the disease Aids, and scientists say knowledge of its origins could help find remedies.
"HIV-1 group M subtype B" predominates in the US, Europe, large parts of South America, Australia and Japan.
Now scientists say they know where it came from.
The findings are published in the Proceedings of the US National Academy of Sciences.