Ebola, HIV, influenza, MERS. Plenty of animal viruses cause devastating diseases in humans. But nature might have many more in store.

In a new study, U.S. researchers estimate that there are more than 320,000 unknown viruses lurking in mammals alone. One of them could touch off the next pandemic if it jumps to humans, says Nathan Wolfe, a virologist who was not involved in the work and founder and chief executive office of Metabiota, a company that contracts with governments and health agencies to track disease outbreaks. “This paper gives an idea of what’s actually out there.”

Scientists estimate that almost two-thirds of emerging infectious diseases originate in wild animals, such as birds, bats, primates and rodents.

Bats in particular have been in the spotlight recently as they are suspected to be the reservoir for many deadly viruses such as Ebola, severe acute respiratory syndrome (SARS) and Nipah, which can lead to inflammation of the brain (encephalitis) or respiratory disease. Some scientists argue that bats’ immune systems may make them more likely to pass pathogens to humans.

To estimate how many viruses might be lurking in wild mammals, researchers from Columbia University and EcoHealth Alliance, a conservation organization also in New York, studied flying foxes in Bangladesh. From 2006 to 2010, they caught hundreds of the big bats and collected urine and fecal samples as well as throat swabs before releasing them. They then fished out all the viral sequences they could find belonging to nine virus families, including the coronaviruses, herpes viruses and influenza A.

They found 55 viruses in all, 50 of which had never been seen before, including 10 in the same family as the Nipah virus that has caused numerous outbreaks in South Asia since surfacing in 1999.

“I think it represents a new period we are entering in terms of these viral discovery studies,” Wolfe says.