Researchers in France have found a new strain of the virus the causes AIDS.
In a report in the journal Nature Medicine, the researchers identify the patient as a 62-year-old woman from the African nation of Cameroon who tested positive five years ago.
They say the strain she has differs from the three known strains of human immunodeficiency virus and appears to be closely related to a form of simian virus recently discovered in wild gorillas.
The researchers say the finding "highlights the continuing need" for experts to keep a close watch for new HIV variants, particularly in western central Africa.
The three previously known HIV strains are related to the simian virus that occurs in chimpanzees.
The researchers say the most likely explanation for the new find is gorilla-to-human transmission. The woman is said to have had no contact with apes or bush meat, and the researchers say the strain could be circulating unnoticed in Cameroon or elsewhere.