Deep in the rainforest of south-east Cameroon, the voices of the men rang through the trees. "Where are the white people?" they shouted. The men, who begin to surround us, are poachers, who make their money from the illegal slaughter of gorillas and chimpanzees. They disperse but make it known that they are not keen for their activities to be reported; the trade they ply could not only wipe out critically endangered species but, scientists are now warning, could also create the next pandemic of a deadly virus in humans.

Eighty per cent of the meat eaten in Cameroon is killed in the wild and is known as "bushmeat". The nation's favoured dishes are gorilla, chimpanzee or monkey because of their succulent and tender flesh. According to one estimate, up to 3,000 gorillas are slaughtered in southern Cameroon every year to supply an illicit but pervasive commercial demand for ape meat .

"Everyone is eating it," said one game warden. "If they have money they will buy gorilla or chimp to eat."

Frankie, a poacher in the southern Dja Wildlife reserve who gave a fake name, said he is involved in the trade because he can earn good money from it, charging around £60 per adult gorilla killed. "I have to make a living," he said. "Women come from the market and order a gorilla or a chimp and I go and kill them."

Cameroon's south-eastern rainforests are also home to the Baka – traditional forest hunters who have the legal right to hunt wild animals, with the exception of great apes.

Felix Biango, a Baka elder, said the group used to hunt gorilla every few weeks to feed his village, Ayene, but has stopped since Cameroon outlawed the practice 10 years ago. However, he says that every week, three or four people come from the cities to ask the group to help them to hunt wild animals, such as gorillas and chimpanzees.

While the Baka no longer hunt primates for themselves, Mr Biango says that they still kill gorillas for the commercial trade and will eat the meat if they find the animals already dead.

Though Cameroonians have eaten primate meat for years, recent health scares have begun to raise fears about the safety of the meat. "In the village of Bakaklion our brothers found a dead gorilla in the forest," Mr Biango said. "They took it back to the village and ate the meat. Almost immediately, everyone died – 25 men, women and children – the only person who didn't was a woman who didn't eat the meat."

Three-quarters of all new human viruses are known to come from animals, and some scientists believe humans are particularly susceptible to those carried by apes. The human immunodeficiency virus (HIV) is now widely believed to have originated in chimps. Apes are known to host other potentially deadly viruses, such as ebola, anthrax, yellow fever and other potential viruses yet to be discovered.

Babila Tafon, head vet at the primate sanctuary Ape Action Africa (AAA), in Mefou, just outside the capital Yaounde, believes the incident that Biango describes could have been caused by an outbreak of ebola, but cannot be sure because no tests were carried out.

AAA now cares for 22 gorillas and more than one hundred chimps – all orphans of the bushmeat trade.

Mr Tafon tests the blood of all apes arriving at the sanctuary. He says he has recently detected a new virus in the apes – simian foamy virus, which is closely related to HIV. "A recent survey confirmed this is now in humans, especially in some of those who are hunters and cutting up the apes in the south-east of the country," he said.

Viruses are often transferred from ape to human through a bite, scratch or the blood of a dead ape getting into an open wound. There is a lower risk from eating cooked or smoked primates, but it is not completely safe.

Bushmeat is not only a concern for Cameroonians. Each year, an estimated 11,000 tons of bushmeat is illegally smuggled in to the UK, mainly from West Africa, and is known to include some ape meat.

The transfer of viruses from ape to man is a primary concern for the international virology research and referral base run by the Pasteur Centre in Yaounde. Each week, it screens more than 500 blood samples for all manner of viruses, and alerts major international medical research centres if it finds an unfamiliar strain.

Professor Dominique Baudon, the director of the Cameroon centre, says he is concerned that the bushmeat trade is a major gateway for animal viruses to enter humans worldwide, due to the export trade.

He says that the deeper poachers go in to the forest, and the more that primates are consumed, the more exposed people become to new unknown viruses and the more potential there is for the viruses to mutate into potentially aggressive forms. At the Ape Action Africa sanctuary, Rachel Hogan, who came to Cameroon from Birmingham 11 years ago, and her team focus on the last of Cameroon's great apes.

It is not known exactly how many gorillas remain in the wild in Cameroon. Conservationists estimates there may be only a few thousand Western Lowland Gorillas left, which are being gradually forced in to smaller groups by hunting and the destruction of their habitat by logging. In the west of the country, there are only 250 Cross River Gorillas left.

Hunting does not just affect adult apes. One hunter said a baby gorilla had screamed so much for its dead mother, killed for her meat, that he eventually killed it to stop the noise.

Most of the gorillas and chimps Ms Hogan and her team look after are babies who have witnessed the murder of their parents. She says they are often suffering from terrible wounds and even trauma when they arrive at the sanctuary. "They grieve just like humans," she says. "We have had them where they will just sit rocking, grinding their teeth and they don't respond to anything. You have to be able to win back their trust."

Ms Hogan says the apes can even die after the trauma. "They'll stop eating, they won't respond to anything... [They] decide whether they live or die. It's like watching a clock wind down."

The increasing number of rescued apes is putting pressure on the sanctuary. A group of eight gorillas in the wild, protected by one dominant male, needs 16 square kilometres to roam in to live comfortably.

The sanctuary says there is nowhere in the vast tropical rainforest of Cameroon that the apes can safely be returned to the wild. "If this continues there might not be any wild populations of gorillas left," says Ms Hogan.

'Unreported World: The Monkey Business', Channel 4, 7.30pm tonight

Out of Africa: How HIV was born

Aids, the worst pandemic of modern times which has claimed over 30 million lives, is thought to have begun in the rainforest of west central Africa as a result of the bush meat trade.

For decades, perhaps centuries, wild chimpanzees carrying the Simian Deficiency Virus (SIV) have come into contact with humans who have caught and eaten them. SIV is genetically similar to HIV and, occasionally, when a chimp scratched or bit a hunter, the virus will have been passed on and may have mutated into HIV. In the distant past, when communications were poor, outbreaks of HIV would not have spread beyond the forest. But in the latter part of the last century, as the commercial exploitation of Africa gathered pace, the opportunities for viral spread increased.

Today, the scale of the slaughter is immense. The Washington-based Bush Meat Crisis Task Force estimates that up to five million tons of wild animals are being "harvested" in the Congo Basin every year – the equivalent of 10 million cattle. The trade was initially driven by hunger – it was a cheap source of food – but has burgeoned with increased logging of the forests and growing demand.

Now, it is international, extending the threat beyond the continent's boundaries. Scientists have warned that Britain is at risk from an outbreak caused by the lethal Ebola or Marburg viruses contained in illegal imports of bush meat from Africa.

The size of the imports is unknown, but one 2010 study estimated that five tons of the meat per week were being smuggled in personal baggage via Roissy-Charles de Gaulle airport in Paris, France. Gorilla and chimpanzee meat is said to be on offer to African communities in Hackney and Brixton at hundreds of pounds per kilogram.