This is the largest outbreak of Ebola in history and the first in west Africa. The disease is transmitted by body fluids, and may have originated in bats. Official figures suggest that almost 5,000 people have been infected and more than 2,400 have died, mainly in Liberia, Sierra Leone and Guinea. These figures underestimate the problem, given that many patients are thought to be unaccounted for. Roughly half of those who become infected go on to die.