The outbreak of Ebola in the Democratic Republic of Congo is concerning. It has been spreading for weeks undetected in a part of the world where civil war makes getting on top of the virus difficult, and the species of Ebola involved is rare, so there are fewer tools to stop a virus that kills between a third and half of people infected. This is a critical moment in an outbreak where there is uncertainty about how far it has spread, but there are already almost 250 suspected cases and 80 deaths.