A new strain of HIV has been discovered in France, and most worrying is the fact that it may not be detected on standard AIDS tests. The new variety of human immunodeficiency virus appears to have originated in a gorilla disease that was first isolated in 2006.

The three previously known HIV strains were related to a chimpanzee illness. Experts say that the new strain should still be treatable with the same anti-retroviral drugs that help curb the other strains of HIV.

Researchers indicated that they found the new strain in a 62-year-old woman who tested positive in 2004, shortly after she moved to Paris from Cameroon. The woman is not sick, and had lived in a suburb of the Cameroonian capital and had no contact with apes or wild animal meat. She did, however, indicate that she had several sexual partners. Researchers said the likely contracted the new HIV strain from another person infected with it.

Writing in the magazine Nature, researcher Jean-Christophe Plantier of the University of Rouen said that the discovery "highlights the continuing need to watch closely for the emergence for new HIV variants, particularly in western central Africa, the origin of all existing HIV-1 groups."

Professor Paul Sharp of the University of Edinburgh told the BBC that the virus may have jumped from gorillas to chimps to humans. He added:

"The medical implication is that, because this virus is not very closely related to the other three HIV-1 groups, it is not detected by conventional tests.

The original strain of HIV is believed to have jumped from apes to humans about a century ago, from either the bite of an infected animal, or a person killing and eating a sick chimp. Analysis of tissues perserved by doctors in colonial-era Belgian Congo show that HIV-1 began to spread among humans between 1884 and 1924.

In humans, HIV is a pandemic and without treatment it usually leads to AIDS and eventually death. The United Nations estimates that 33 million people worldwide are currently infected, and since the 1980's AIDS has killed more than 25 million. The disease is most prevalent in sub-Saharan Africa. Elsewhere it is common amongst sex workers, those who inject drugs, and gay men.