In 2015, a mysterious virus, eventually identified as Zika, first appeared in Brazil, flooding clinics with patients. The year before, the world watched in horror as the Ebola virus tore through the heart of West Africa, leaving some 12,000 dead in its wake. We are familiar with the daily headlines of the epidemic's unfolding saga. But behind those headlines is an untold story that could change the way we fight future outbreaks. Over the last half-century, the number of ‘spillover’ diseases has increased rapidly. Why? We follow scientists and experts into the world's hot zones in a search for answers. The quest provides much-needed scientific context for the most recent Ebola and newly unfolding Zika crises, as battle-hardened health workers and survivors share their previously untold stories.