Disease X n. A dire contagion requiring immediate attention—but which we don’t yet know about. In 2013 a virus jumped from an animal to a child in a remote Guinean village. Three years later, more than 11,000 people in six countries were dead. Devastating—and Ebola was a well-studied disease. What may strike next, the World Health Organization fears, is something no doctor has ever heard of, let alone knows how to treat. It’s come to be known as Disease X.