Understanding where new viruses come from is critical for preventing them from rapidly spreading among humans. When it comes to preventing the next pandemic, a new study suggests that bats may be public enemy number one.

In a new study published in the journal Nature, researchers at the nonprofit EcoHealth Alliance collected data on viruses known to infect mammals, which included about 600 viruses found in more than 750 species. They were then able to calculate the number of viruses from each species and identify characteristics that make the transmission to humans more likely. Living more closely to humans and being more closely genetically related to humans increased the odds of transmission.

Out of all the species assessed, bats carried the highest number of these viruses. Researchers are currently looking into why.

“A lot of people don’t realize that these viruses have been on the planet for a long time, and they are in populations of animals all around the world,” says study author Kevin Olival, associate vice president for research at EcoHealth Alliance. “What we did in this study is prioritize where we should look if we want to stop the next Ebola or Zika from emerging.”

All groups of mammals were found to carry viruses that can spread to humans, and areas around the world most at risk for carrying emerging viruses differed based on the mammal. For bats, these places are most common in South and Central America and areas in Asia. For primates, the areas with the higher risks are in Central America, Africa and Southeast Asia.

The study was funded as part of the United States Agency for International Development (USAID) Emerging Pandemic Threats PREDICT program, a project that seeks to identify new emerging infectious diseases that could become threatening to human health. Olival says his team hopes that scientists will use this research to identify regions and viruses to focus on for prevention efforts.

Bats don’t deserve all of the blame, however. The spreading of new diseases often involves activity from both animals and people, Olival points out.

“These diseases are not just randomly jumping into people,” he says. “We see time and time again that it is the human disturbances in the environment that are causing these diseases to emerge,” through activities like chopping down forest and hunting animals out of certain areas. “It is our interactions with these species that are causing diseases to jump.”