The question has stumped scientists since the Sars virus first emerged in China and spread around the world: where did it come from?
Now, researchers in Hong Kong have identified the culprit - the masked palm civet, a small cat-like mammal that is treated as a culinary delicacy in some parts of China.
Virologists brought in to hunt down the Sars virus always suspected that it had lingered in farmyard or domestic animals before making the cross-species jump into humans.
The most likely sources were believed to be either pigs or chickens, but attempts to infect the animals with the Sars virus taken from infected humans failed, suggesting that it must have come from another species.
Yuen Kwok-Yung, a microbiologist at the University of Hong Kong, and his team decided to screen large numbers of civets and other game animals. They found that four of the masked palm civets - which have short fur and black and white stripes on their faces - were carrying a coronavirus that caused Sars. The animals are common in China, India and Malaysia.
Prof Yuen said it was unlikely that people had been infected by eating civets. But the virus may have jumped into humans as they raised, slaughtered and cooked them.
Civet is one of the main ingredients in the exotic wildlife dish "dragon-tiger-phoenix soup", for which wealthy Chinese in Guangdong province will pay large sums.
The soup is flavoured with chrysanthemum petals and includes shreds of civet cat and snake.
The current website of Guangzhou's tourist information centre invites visitors to sample this "special dish".
The sale of civet cat is banned in Hong Kong, but people still cross into China to eat it, and other exotic animals. It is widely believed in southern China that eating wildlife can increase the vigour of human organs.
A Hong Kong virologist, Professor Malik Peiris, who heads the team which first identified the Sars coronavirus, said yesterday that as long as the food was properly cooked, the virus should be killed.
However, he said, it was obviously being shed from the wild animal, and someone could come into contact with it while the live animal was being handled.
One of China's first confirmed Sars patients, Huang Xingchu, 34, worked as a cook in a Shenzhen restaurant.
Prof Yuen said strict controls should be put in place to ensure civets and other game animals were raised and sold on without putting people at risk of infection.
"If you cannot control further jumping of such viruses from animals to humans, the same epidemic can occur again - so it is very important that we have ways of controlling the rearing, the slaughtering and the selling of these wild game animals."
"This is a good step forward," said John Oxford, an expert in virology at Queen Mary School of Medicine in London. "Now we can start putting barriers between the people and the cats to stop it, and further viruses, coming across."
Earlier this week, Chandra Wickramasinghe, of Cardiff's Centre for Astrobiology, suggested the Sars virus may even have fallen to Earth from space, but this is not a view widely shared by scientists.
Prof Yuen and his team believe the virus jumped straight from civets to humans, but say other animals may also have been involved in transmission.
"It may not be the end of the story," said Professor Oxford. "It might well have jumped from another animal into the civets, and it will take a lot of work to sort that out."
The Sars epidemic is believed to be coming under control in Hong Kong, and yesterday the World Health Organisation withdrew its warning against travel to Hong Kong and Guangdong.
Taiwan is now reporting the highest number of new infections, bringing the total for the region to 483. Worldwide, 4,212 people remain infected with the disease.