Bats are creatures of the night that are commonly held in fear. At first glance, those fears might seem to have some medical justification. Long known as vectors for rabies, bats may be the origin of some of the most deadly emerging viruses, including SARS, Ebola, Nipah, Hendra and Marburg. Instead of demonizing bats, however, research shows the real culprit behind these outbreaks could be human error.

The Nipah and Hendra viruses were the first emerging diseases linked to bats. Hendra claimed two of its three victims in its first and so far only known appearance in Australia. Meanwhile Nipah has in repeated Southeast Asian outbreaks killed nearly 200 people, and blood tests of wildlife have suggested that the viruses came from the largest bats, flying foxes.

The connection to SARS, or severe acute respiratory syndrome, was less direct. During the outbreak that began in China in 2002, investigators found that civets and two unrelated species harbored the SARS coronavirus, prompting mass culling of the mongooselike civets by the Chinese government. Subsequent research, however, found no widespread SARS infection among wild or farmed civets, indicating that the disease arose in another species and might remain in wait there.

From research with Nipah and Hendra, virologist Linfa Wang of the Australian Animal Health Laboratory knew bats could get chronic infections from the viruses while not getting sick, making them ideal carriers for disease. Bats, civets and a menagerie of other animals were often found caged near one another in live-animal markets in Asia. So Wang hypothesized that bats might harbor SARS as well.

Wang and his colleagues analyzed blood, throat and fecal swabs from 408 wild bats from China. Genetic analysis revealed five bats, which represented three of nine species of horseshoe bats tested, possessed viruses closely related to SARS. They reported last September that the genetic variation within those coronaviruses was far greater than that seen in human or civet SARS. Therefore, bats, probably having lived longer with the diseases, may be the origin of the coronaviruses seen in other species.

Then, in December, researchers connected fruit bats to Ebola, whose origin in the wild had remained unknown since its first recorded appearance 30 years ago. During the Ebola outbreaks in humans, gorillas and chimpanzees between 2001 and 2003 in Gabon and the Republic of the Congo, a team led by virologist Eric M. Leroy of the International Center of Medical Research in Franceville, Gabon, tested some 1,000 animals. Of 679 bats studied, 16 had antibodies against Ebola, and 13 others possessed Ebola gene sequences in their liver and spleen. The sequences demonstrated genetic diversity, "indicating that Ebola probably has spent a long time within bats, suggesting that bats might be the origin," Leroy says. Virologist W. Ian Lipkin of Columbia University notes that scientists suspect that the Marburg virus, a relative of Ebola, also originated in bats.

Leroy vigorously argues that bats should not be culled. Wang agrees, observing that bats play critical ecological roles, such as eating insects and other pests. Besides, Wang points out, culling is simply not practical when it comes to bats, which can just fly away. Satellite collars on fruit bats carrying Nipah showed they could fly between Thailand, Sumatra and Malaysia, and the horseshoe bats linked with SARS range across Asia, Europe and Australia.

Preventing future emergences may instead focus on human behavior. Just as SARS is potentially linked to animal markets, so was Nipah linked to pigpens encroaching on bat habitats. And people living in Ebola-endemic areas eat the bats harboring the virus. Knowledge that bats can carry dangerous viruses could work to prevent epidemics, notes Peter Daszak, executive director of the New York City-based Consortium for Conservation Medicine, which studies the connection between emerging diseases and human interactions with the environment. Keeping bats from the wildlife trade might have dramatically cut the risk of SARS emerging, perhaps saving $50 billion worldwide in loss to travel, trade and health care costs "and hundreds of lives," Daszak says.