The viruses, which are not currently linked to any disease, were discovered in Cameroon among primate hunters, according to researchers from Johns Hopkins University in Baltimore, Maryland and the US government's Center for Disease Control and Prevention.

The two new strains, "Human T-lymphotomic Virus type 3 and 4" (HTLV-3 and , were probably transmitted to humans via bites or other wounds, they said. The 12th annual conference on retroviruses wrapped up here Friday.

Scientists generally believe that human immunodeficiency virus or HIV, which belongs to another retrovirus group, also comes from monkeys, and that the virus mutated and spread among humans through sexual contact.

CDC virologist Dr Walid Heneine said research was underway in Cameroon to evaluate the health of the infected persons and to find their sexual partners. There has been no evidence of transmission of the viruses among humans so far, he said.

"The appearance of these retroviruses in humans demonstrates that cross-species transmissions may be frequent. To date there is no evidence of human to human transmission," Heneine said.

The first two types of viruses of the HTLV group, of which a total of four are known, are responsible for serious diseases. HTLV-1 causes leukemia and inflammation, while HTLV-2 can cause neurological problems, the scientist said.

HTLV-1 and 2 infect about 22 million people worldwide, according to estimates, and about five percent develop a disease, according to the CDC.

As with AIDS, several years can elapse between infection and the onset of disease.

Researchers involved in the Cameroon discovery began by studying people who have contact with monkeys, such as veterinarians in zoos, then expanded their study to a group of 930 Cameroonians in frequent contact with monkey meat.

Participants in the study responded to a questionnaire and gave a blood sample.

Upon identifying the first known human HTLV-3 infection, researchers realized it was genetically identical to the simian STLV-3 virus.

Eleven other Cameroonians participating in the study were infected with HTLV-1, similar to a primate virus.

The researchers plan to conduct additional tests on a group of 4,000 people living in rural areas of Cameroon to evaluate the extent of an eventual transmission of the retroviruses among humans.

Monkeys have apparently never developed AIDS, which has led scientists to study their immune systems in an attempt to figure out what protects them.

Scientists last year discovered a protein called TRIM5 alfa that blocks HIV from entering a simian cell, and are trying to figure out why the same protein does not play a similar role in humans.

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