In 2016, just as the worst Ebola epidemic in history was dying down in west Africa, researchers from a US government-funded pandemic-surveillance program called PREDICT sampled bats in the hardest hit region in search of Zaire ebolavirus, the virus responsible for the outbreak. They were looking for animal hosts from which the epidemic had sprung.

Although they didn’t manage to do that, they found something else: a new species of ebolavirus, the genus of viruses that cause Ebola diseases. The new virus—the sixth in the genus to be identified—infects two bat species that roost in people’s homes in the Bombali region of Sierra Leone. The following year, the new virus popped up in the same bat species in Guinea and Kenya.

Although researchers don’t yet know whether this new Bombali virus infects people, or whether it would cause disease if it did, lab experiments suggest that it could. The protein that helps the Bombali virus, and the other five ebolaviruses, enter host cells binds to the correct receptor in lab-grown human cells to gain access to them, explains Tracey Goldstein, associate director of the One Health Institute at the University of California, Davis, who led the team that identified the new virus.

The Bombali virus highlights an enigma for scientists monitoring the viruses that jump from wildlife to humans, a process called zoonotic spillover. Ebolaviruses, and many other viruses circulating in bats and other animals, “have that ability to spill over all the time,” Goldstein says. “But they don’t spill over all the time.”

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