A few years ago, disease ecologist David Hayman made the discovery of a lifetime.
He was a graduate student at the University of Cambridge. But he spent a lot of that time hiking through the rain forest of Ghana, catching hundreds of fruit bats.
"We would set large nets, up in the tree canopies," he says. "And then early morning, when the bats are looking for fruit to feed on, we'd captured them."
Hayman didn't want to hurt the bats. He just wanted a few drops of their blood.