When SARS emerged late in 2002, killing hundreds of people as it leapfrogged its way around the globe, infectious disease experts rushed to try to find out what kind of bug was causing the fatal respiratory infection. They also asked: What animal did it come from?

That's because three quarters of new infectious diseases in humans originate in animals. Understand what animals carry it, and it's easier to halt the spread of the disease in humans.

For example, new strains of influenza usually arise in birds or pigs before they're passed on to humans. Researchers are racing to track the spread of H5N1 avian influenza in wild birds, hoping to be able to head off a global flu pandemic among humans. And scientists are still tracking cases of West Nile virus in birds and mosquitoes in the United States, to have a better idea of how the virus makes humans sick and how best to prevent infection, whether by simple measures such as mosquito repellent or by altering habitats to reduce the number of infectious critters.

This week, researchers in China, Australia, and the United States said they'd tracked the source of the coronavirus that causes SARS to bats in Southern China. That comes as a bit of a surprise, because in 2003 the virus had been discovered in civets, catlike wild animals that are sold as exotic food in "wet markets" in Guangdong province, where SARS first showed up.

Chinese authorities killed hundreds of civets in an effort to halt the spread of the disease. But testing later showed that most civets weren't infected with the SARS virus, and when civets were exposed in experiments they got quite sick, suggesting that, apparently lacking in immunity, the animals weren't the natural reservoir for the disease.

Enter bats. They, too, are sold in wet markets for food, and bat parts are used in traditional medicine. The small mammals also harbor Hendra and Nipah viruses, two new bugs that caused fatal outbreaks among humans in Australia and East Asia in the 1990s. That was enough to raise suspicion among researchers, who trapped bats in the wild in China in 2004 and tested them for antibodies to the SARS virus.

Three species of cave-dwelling bats were found to have been exposed to SARS-like viruses, with 71 percent of the bats in one species testing positive. By comparing DNA from the SARS-like bat viruses to the ones from humans and civets, the scientists were able to determine that the bug almost certainly originated in bats.

"You can start to look at how the virus has evolved," says Peter Daszak, executive director of the Consortium for Conservation Medicine, a coauthor of the study. "The virus is present in a small number of bats. It either gets into humans or into other animals in the wild trade. And it's pretty clear that that event happened just before the first [human infections] in China in 2002." The report is published in Science.

The next step: figuring out how to prevent the spread of SARS from bats to humans. Daszak points to Malaysia as an example of how animal-to-human transmission of fatal infections can be fought. After an outbreak of Nipah virus killed 100 people in 1999, authorities imposed restrictions on planting trees favored by fruit bats, in order to reduce the risk of another outbreak.

"SARS cost globally over $50 billion," says Daszak. "There's a strong argument to say that if we do a little prevention right now, we save human lives and it's very cost effective."