The Ebola crisis now under way in West Africa is the biggest outbreak of the virus ever, but it is not the first—it is the 23rd since the disease arose in 1976 (polygons below). The scourge began in Guinea, distant from earlier cases (map), and is by far the largest (bottom right corner below). This analysis by University of Oxford researchers of previous outbreaks shows that the death rate has been greater than 33 percent and that there is often more than one cluster of infections, typically because of sick individuals seeking treatment or traveling. Experts worry that without vigorous, sweeping efforts to identify and suppress new outbreaks as soon as they emerge, the virus will become a permanent health risk, erupting unpredictably in Africa and around the world.