The virus that causes SARS, or Severe Acute Respiratory Syndrome, may have originated as an animal virus. Other diseases, including AIDS and Ebola, also jumped from animals to humans. Experts say southern China is considered especially fertile ground for diseases to jump species.
In southern China, large numbers of people share living space with their livestock. There are also rodents and other wildlife around. Veterinarian Neils Pedersen at the University of California at Davis, says, pack them all together, and you've got a recipe for trouble.
"The greater numbers of these various things you bring together, and the more closely you bring them together, the greater the chance of these events happening," he said. "It's just almost the perfect pressure cooker, if you want to call it that, or the perfect kitchen to cook up one of these things."
Human coronaviruses, like the one that causes SARS, usually cause only the common cold. Some other members of the coronavirus family can cause more serious diseases in pigs, chickens, cows, cats, dogs, and other animals. These viruses do not normally infect people.
But, if people live in close quarters with animals, there are more opportunities for people to be exposed to animal viruses. And that can lead to changes, according to Doctor Kenneth McIntosh, of Harvard University.
"If two coronaviruses somehow infect the same cell, they can exchange genetic material and you can get daughter viruses or son viruses that have elements of both parents," he said.
A virus that infects pigs, for example, could suddenly acquire genetic material that lets it infect people.
Dr. McIntosh was part of a group that first discovered that coronaviruses can cause colds in humans. He says coronaviruses like the SARS virus do not jump species as easily as flu viruses do. Several major flu outbreaks this century have been blamed on viruses that came from animals in southern China. Most recently, "bird flu" killed six people in Hong Kong in the winter of 1997-98. Authorities ordered millions of chickens slaughtered to contain the disease.
Elsewhere in the world, other viruses have also made the jump. HIV, for example, probably came from chimpanzees and monkeys in Africa. Bacteria and fungi can also cross over.
Dr. McIntosh says these diseases that are new to humans can be especially dangerous. "If humans have not had it before, then they can be pretty guaranteed not to have any immunity to it," he said. "So if it does spread from human to human, it will have a field day. And that is what people are so worried about with SARS."
Dr. McIntosh says not all germs that jump from animals are as severe as SARS. Some may infect people and cause only mild symptoms.
Kathryn Holmes, who studies coronaviruses at the University of Colorado, says if an outbreak of a new disease can be contained, it may burn itself out and never reappear. "It's possible that a virus mutant developed in the animal that could infect people but had lost its ability to infect its original host. In that case, you would have no animal reservoir for this particular mutant," she said.
Ms. Holmes says without that animal reservoir, where the virus can survive until it finds another human host, the disease may simply disappear.
But scientists don't know enough about SARS to say whether that will happen. They say SARS may be here to stay.
Even if SARS does disappear, scientists say other diseases will probably emerge from southern China in the future.
But Mr. Pedersen, the veterinarian, says people can reduce the chances of diseases jumping species by keeping their livestock away from people, and controlling livestock diseases. Rodent control may also help. But, he says, nothing will entirely stop diseases from jumping species.
"This is a natural process that's always occurring because that's what DNA and RNA does, it evolves. If you could wave a magical wand and say, 'no more evolution, no more DNA changes,' yeah, you could stop all these things. But that's not going to happen," he said.
So public health officials will have to stay alert for the next new disease.