Scientists searching for the origin of HIV, the global pandemic infecting more than 40 million people, believe they have finally tracked its original source to two colonies of chimpanzees in a corner of Cameroon.

The finding represents the culmination of a 10-year hunt for the source of the pandemic and provides a crucial link between HIV, which causes Aids in humans, and the simian immunodeficiency virus (SIV), a strikingly similar virus that infects monkeys and chimpanzees. Researchers believe that south-east Cameroon is where the virus first jumped from chimpanzees to humans before HIV infection began spreading among people as far back as the 1930s.

At the time, HIV, which destroys the immune system leaving those infected vulnerable to myriad diseases, was difficult to diagnose. Carried by people travelling along the rivers, it spread unnoticed to Kinshasa, where the first human epidemic began to grow.

Researchers at Nottingham University joined scientists in Montpellier and Alabama to search for signs of the virus in 10 chimpanzee populations throughout Cameroon, where SIV was known to be circulating.

Cameroon is home to two different subspecies of chimpanzee, separated by the natural border of the Sanaga river which slices the country into north and south. North of the river, tests on chimpanzee faeces found no traces of SIV, but it was present in colonies immediately on the southern side.

As the search for the source of HIV moved further south, researchers located two colonies, living deep in the south-east corner of the country near the Ngoko river bordering the Democratic Republic of Congo.

Tests on chimpanzee faeces here revealed an SIV strikingly similar to the HIV that causes Aids in humans. In some communities they detected SIV specific antibodies and viral genetic information in as many as 35% of chimps.

"For us, this is really the last piece of the puzzle," said Paul Sharp, a professor of genetics at Nottingham University. "This is where it probably all started. We've got these viruses in south-east Cameroon, which are so close to HIV, and it's difficult to envisage there could be any which could be closer." The research appears in the journal Science today.

"There was a nagging doubt that chimps had picked it up from monkeys, because monkeys carry the virus and chimps sometimes eat them," he added. "We didn't know for sure if humans picked up the virus in parallel from the same type of monkey. No one had really shown if there was a reservoir of this virus out there in the wild."

Researchers have various theories on how the virus jumped to humans; the most widely held view is that hunters became infected when they caught and butchered infected chimps. SIV appears to cause no outward signs of illness, so hunters would not have known if they were catching infected animals.

"When people hunt chimpanzees they tend to butcher them on the spot and then there's a lot of blood flying around," said Prof Sharp. "If the hunter has any open wounds then, that's an opportunity to get infected.

"Chimps and humans are extremely similar genetically, but here we have a virus that is seemingly harmless in chimps, jumps into humans and suddenly causes Aids."

Researchers believe the virus infected humans some time before the 1930s and was gradually spread by river travel. All of the rivers in Cameroon run into the Sangha, which joins the Congo river running past Kinshasa.

Trade along the routes could have spread the virus, which slowly built up in the human population.

For SIV to cause a pandemic in humans, it must have first mutated into a form that could readily be picked up from infected chimpanzees. Then it adapted until it was infectious enough to be passed from person to person.

The first clearly identified case of Aids reported in the United States was in 1981, though it seems an African American teenager died of it in St Louis in 1969.

Epidemiologists who have studied the growth of the pandemic suggest that by the 1960s about 2,000 people in Africa may have had HIV, which was as yet unnamed. By 1980, closer to 1 million people were infected, and the virus was named a year later.

OF the 40 million people living with HIV now, more than two-thirds are in sub-Saharan Africa, which is also home to 77% of the women with HIV.

Last year, there were nearly 5m new HIV infections, 4.2m among adults and 700,000 in children under 15. In the same year, there were 3.1m deaths from Aids, more than half a million of whom were children.

Identifying the source of the HIV pandemic is more than filling in a missing link in the disease's progression. Understanding how the virus infects chimpanzees, and why it does not appear to cause disease in such a similar genetic species, could unveil useful clues about how the virus works in humans and which crucial changes happened to the virus when it made the leap to humans.