Blood samples
 
Scientists undertook a genetic analysis of stored blood samples from early Aids patients to discover from when the virus came to the US.
 
They found that HIV was brought to Haiti from Africa around 1966 by an infected person, and subsequently arrived, probably in a major city, in the US around 1969.
 
It is thought that from here it circulated for many years through the US and to other nations.
 
The team used blood samples from Haitian immigrants with a mystery illness stored by the US government as early as 1979.
 
Five samples dating from 1982 and 1983 were analysed, as was genetic data from 117 early Aids patients from different parts of the world.
 
From this the molecular clock of the world's predominate strain of HIV was calibrated and subsequent calculations gave an estimate for the disease's arrival in Haiti from Africa and then in the US.
 
'Hard evidence'
 
Dr Arthur Pitchenik, from the University of Miami, is one of the study's authors, which was published in the journal Proceedings of the National Academy of Sciences.
 
He said: "I think that it gives us more clear insight into the history of it [the Aids epidemic] and what path the virus took - and hard, objective evidence, not just armchair thinking."
 
The study gave a 99.8 per cent probability that Haiti was the disease's stopping point, rather than HIV moving directly between Africa and the US. The disease's path between countries has been debated for a long time.
 
The leader of the study Michael Worobey, of the University of Arizona, Tucson, said the date of 1969 for Aids' arrival in the US is earlier than some experts expected and around 12 years before the disease was discovered in 1981.
 
During this time HIV infections were rising and people were dying of the disease.
 
"It is somehow chilling to know it was probably circulating for so long under our noses," Worobey said.