The AIDS virus entered Britain at least six separate times in the early and mid-1980s, not once as has been widely believed, researchers reported on Monday.

The team lead by University College London researchers found that HIV-1 subtype B, the most common HIV-1 subtype found in Britain, spread quickly via at least six large transmission chains of men having sex with men.

There seems to be no geographic center for any of the epidemics, suggesting that men who carried the virus moved around the country, the researchers said in the report, published in the Proceedings of the National Academy of Sciences.

The virus was passed on less often in the 1990s - most likely because men began using condoms, they added.

HIV, which causes AIDS, affects 40 million people worldwide, with 14,000 new infections every day. There are at least six different subtypes.

More than 57,700 people in Britain have been infected. The B subtype is transmitted mostly among men having sex with men. Elsewhere, HIV is most commonly transmitted among heterosexuals.

“Our study suggests that the HIV-1 subtype B epidemic currently circulating the UK is made up of at least six established chains of transmission, introduced in the early and mid 1980s,” Dr. Deenan Pillay of University College London’s Center for Virology said in a statement.

“This goes against the prevailing belief that one initial entry of HIV-1 was responsible for the spread of the epidemic.”

For their study, Pillay and colleagues created a genetic family tree for HIV in Britain using samples taken from 1,645 British patients and 1,784 samples of subtype B from around the world.

Pillay said condom use may account for the slowed rate of new transmissions, and said his team found no evidence that HIV drug treatments, which can suppress the virus and keep patients relatively healthy, had much of an effect on its spread.