In 2014, as the world watched in horror, the Ebola virus tore through the heart of West Africa, leaving some 12,000 dead in its wake. The outbreak was historic: ten times worse than all other Ebola outbreaks combined. Yet beyond the headlines, out of the spotlight, even greater threats were unfolding at the same time: legions of viruses were continuing their march around the globe, largely unreported. Nipah. Chikungunya. Zika. These are viruses —like Ebola—that reside in animals and spill over into humans. Over the last half century, the number of spillover diseases has nearly quadrupled, and produced the AIDS pandemic. What’s behind the rise in spillover viruses? What can we do to stop them?