A new strain of the virus that causes Aids has been discovered in a woman from the West African nation of Cameroon.

The virus strain appears to come from gorillas but researchers said the woman has likely been infected by another human, not an animal.

"We have identified a new human immunodeficiency virus in a Cameroonian woman", Jean-Christophe Plantier of the Universite de Rouen in France wrote together with his colleagues in the journal Nature Medicine on Sunday.

"It is closely related to gorilla simian immunodeficiency virus and shows no evidence of recombination with other HIV-1 lineages..."

The 62-year-old woman had no contact with gorillas but said she had had several sex partners. 

She was diagnosed with HIV in 2004, soon after she moved to France from Cameroon.

Routine genetic sequencing of the virus showed it looked like no other sample of HIV virus and it was eventually compared to a gorilla simian immunodeficiency virus which was discovered in 2006.

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All previously discovered subtypes of the virus have been linked to chimpanzees.

"Our findings indicate that gorillas, in addition to chimpanzees, are likely sources of HIV-1," Plantier's team wrote.

"The discovery of this novel HIV-1 lineage highlights the continuing need to watch closely for the emergence of new HIV variants, particularly in western central Africa, the origin of all existing HIV-1 groups."

There are several theories seeking to explain how SIV, the equivalent of HIV in apes, entered humans.

Scientists say it likely jumped to people hunting chimps. Either an infected ape bit a human, or a SIV-infected animal was killed for bush meat, and the virus entered the
bloodstream of the butcher through tiny cuts in the hand. 

HIV has infected an estimated 33 million people globally and has killed another 25 million.