In December 2013, in the village of Meliandou, Guinea, a dangerous pathogen jumped from a bat into a little boy. He may have been playing among the trees where bats roost, coming into contact with bat guano; the details are uncertain. But several days later, as the pathogen replicated exponentially in his body, the two year old became lethargic, devastated by fever, vomiting, and diarrhea. His mother, older sister, and grandmother cuddled with and cared for the sick toddler. Within days all four of these family members were dead, and the pathogen that had infected them was soon spreading through villages and cities in West Africa.