An Elusive Enemy
No one knows for certain where the Ebola virus lurks between outbreaks; it has never been definitively tracked to a host species in the wild. Transmission to humans has been sporadic and extremely rare—but all too often deadly. Fatality rates in some outbreaks have reached 90 percent. Since Ebola was discovered in 1976, population growth in at-risk countries has nearly tripled, and people have become more mobile.
A Family of Viruses
Besides Ebola itself, which is responsible for most outbreaks, four other viruses are recognized within the ebolavirus group: Sudan, Taï Forest, Bundibugyo, and Reston. All cause Ebola virus disease, except Reston, which is probably harmless to humans. Marburg, a closely related pathogen, was discovered in 1967. Marburg virus disease is also often fatal.
Current Ebola Outbreak
The first outbreaks of Ebola virus disease, in 1976, erupted simultaneously in what was then Sudan (now South Sudan) and Zaire (now the Democratic Republic of the Congo).
Out of the Jungle
Previous outbreaks occurred in sparsely populated forests in central Africa, where Ebola was less likely to spread. The 2014 outbreak in West African cities has infected more people than all previous outbreaks of the past 40 years combined.
*Total confirmed, probable, and suspected cases and deaths as of April 19, 2015. WHO reported the first cases in March 2014.
Jason Treat, Lauren E. James, and Emily M. Eng, NGM Staff; Meg Roosevelt
Sources: WHO; CDC; IUCN; David M. Pigott, University of Oxford; Global Land Cover Facility; LandScan