By Neil Bowdler Science reporter, BBC News

Study's author

The strain passed from Haiti to the US in about 1969 before spreading further, says the team in the Proceedings of the US National Academy of Sciences.

They hope knowing this could help find a cure for HIV, which can lead to Aids.

"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.

'Single carrier'

The team examined archived blood samples from five early Aids patients - all of them Haitian immigrants to the United States - and analysed genetic sequences from another 117 Aids patients from around the world.

With this data, they recreated a family tree for the virus, which they believe shows conclusively that the strain came to the US via Haiti - probably via a single person - in around 1969.

Michael Worobey of the University of Arizona in Tucson is one of the study's authors. He says the new research suggests HIV first arrived in Haiti in the mid-1960s - probably from Africa where HIV is thought to have originated - before making its crossing into the US.

"By 1966 the virus first starts spreading in Haiti," he told the BBC.

"A few years later one variant from Haiti gives rise to what would then light the fuse and explode around the world as the Aids pandemic that we first became aware of."

Prof Worobey and his colleagues now want to trace the strain back further. His suspicion is that it probably arrived in Haiti from the Congo via Haitians who were working in Africa during those years.