The new swine flu has been circulating in animals undetected for a long time, scientists have reported.
A study of the H1N1 strain genes shows that it is only "distantly" related to any other known strains of flu.
Scientists said the strain has eight gene segments and all seem to have come from strains of bird flu – infecting pigs over a period of nearly 100 years between 1918 and 1998.
However six of the eight show signs of coming from more recent viruses – that mix human, bird and pig flu viruses.
The findings, compiled by an international team, have been released by the journal Science. The work was led by Rebecca Garten, of the World Health Organisation, based in Atlanta, Georgia, USA.
They show that the virus has found a new source of virulence compared with existing human viruses.
Researchers said the emergence of the virus suggested a need for greater future monitoring of the virus in pigs.
The World Health Organisation has now recorded some 12,022 cases of the virus worldwide with nine deaths in the USA and 75 in Mexico.
In Britain some 122 cases have been confirmed.
But one expert told a British newspaper that official figures massively underestimate the toll of illness.
According to Professor John Oxford, a senior virologist, thousands have probably contracted the virus but experienced only minor symptoms. Professor Oxford, of Queen Mary College, London, said the true number infected in Britain was probably closer to 30,000.
Leave a Reply