Animal diseases could be the biggest threat to human health in the years ahead, leading scientists have warned.
Some scientists believe Sars passed from animals to humans
Experts at a Royal Society conference in London said there was a growing risk that more viruses will jump the species barrier and infect humans.
Their comments came as global health chiefs linked the death of three people in Vietnam to bird flu.
Officials are culling hundreds of thousands of chickens in an effort to stop the disease from spreading.
In China, thousands of civet cats have been killed after researchers found links between the animal and new cases of Sars.
Professor Nan Shan Zhong, who is based at the Respiratory Disease Research Institute at Guangzhou in China where the first case of Sars was recorded, said he was hopeful the cull would stop the disease from spreading.
"We will have to wait 20 to 30 days to see if there is another peak," he said.
Professor Zhong told journalists that Sars would always be around. He said early diagnosis and treatment were the best ways of containing it.
"Sars will exist for a long time. It will not disappear. It will exist from time to time."
Professor Zhong said there was growing evidence that Sars jumped the species barrier from animals to humans.
He said that while civet cats were the prime suspects other animals might also be involved.
Professor Malik Peiris, a microbiologist at the University of Hong Kong, said animal diseases posed a serious threat to humans not least because so little was known about them.
"Most of these major infection problems are really a problem of animals dumping on humans," he said.
"We really need to know more about the animal-human interface."
Professor Peiris suggested a new flu pandemic, which many experts believe is inevitable, combined with an animal virus could wreak havoc around the world.
He said he was concerned by the deaths in Vietnam.
"It is a cause for great concern. Avian flu is probably a human threat."
Professor Peiris called for more research into animal diseases. But he warned that scientists should examine healthy animals as well as sick animals.
"We need to look at healthy animals," he said. "Diseases that might not harm them may harm humans. This area needs to be looked at. We need better veterinary surveillance."
Professor Tony McMichael of the Australia National University said basic evolution is the reason why animal diseases try to jump to humans.
"Diseases like Sars, HIV and Avian flu have all probably jumped from animals to humans. We really shouldn't be surprised by this.
"We need to think ecologically. These viruses are trying to evolve."
Professor McMichael said there was a long history of animal diseases spreading to humans.
"Animals first passed on microbes to humans thousands of years ago when humans started living in villages.
"Two thousands years ago, humans started living in larger cities. As they met they swapped germs."
He said changes to the way people lived created new opportunities for microbes or diseases to evolve and spread.
"We live in a microbial world. We need to find ways of being a little smarter in how we manage the world around us," he said.
"Whenever we change patterns of human living, human ecology - we are going to create opportunities for microbes.
"We should not be surprised that diseases like Sars and HIV do just that."