In 2015, Brazilian health officials were caught by surprise when they discovered that Zika, a "mild" virus, had caused devastating birth defects in thousands of unborn children. Just months earlier, the world had watched in horror as a familiar foe, Ebola, tore through West Africa, leaving some 11,000 dead in its wake. Zika and Ebola -- like rabies, dengue, and other killer viruses -- reside in animals and spill over into humans. Today, spillover diseases are more common, and we are more vulnerable; in the last half-century the number of spillover viruses may have quadrupled. 

Now, to fight back, scientists are on the hunt for deadly diseases. They hope to stop them before they spill over and spread out of control. Because it's not a question of if another outbreak will strike, but when? And will we be prepared?