So, how did Ebola virus spread away from its normal home range?

Flying bat (image:

NPR spoke to disease ecologist Peter Daszak, who says scientists don't know for sure. But they have a top theory: The virus spread through bats.

Many signs point to bats as the main source of Ebola. Scientists have found Ebola antibodies in bat species that are widespread throughout Africa. The virus infects and replicates inside bats, but it doesn't kill the animals. So bats can easily spread Ebola.

And bats get around. Some can migrate hundreds, even thousands of miles.

"Bats don't need a passport to cross borders," says Daszak, who is the president of the conservation
group EcoHealth Alliance. "Hundreds and thousands of bats migrate across countries. And we've shown that, in other countries, they have really large ranges. And they take the viruses with them."

So does this mean there are giant colonies of bats flying across a large swath of West Africa carrying Ebola?

"It sounds scary," Daszak says. "But bats have a very wide distribution. And people are becoming more and more populous across that distribution. These viruses can emerge anywhere where the wildlife reservoir lives."

There's a couple of big caveats here. This is still a theory. And although scientists have found traces of the virus in individual bats, they still haven't been able to figure out which species are actually spreading Ebola. Or how long the virus has been in West Africa.