How Ebola Works

Ebola Breaks Out

Despite all the fear around Ebola, not that many humans have actually died from the disease, relative to other known diseases. Since 1976, in the 25 outbreaks that have occurred, about 16,000 cases of humans infected with Ebola have been reported and about 6,500 of those cases have ended in death (note: more than 80 percent of the Ebola cases reported are attributed to the 2014 outbreak in West Africa) [sources: World Health Organization], CDC.

As we mentioned, Ebola is actually a family of five virus types. The two most deadly to humans are the Zaire and Sudan types. Not only do these have the highest fatality rates (60-90 percent for Zaire and 40-70 percent for Sudan), they've also appeared the most in the known outbreaks that have occurred [source: Feldmann]. After the initial appearances of both of these strands in 1976, the viruses hid for awhile. But when the Ebola family reappeared, it came back with a vengeance. Since the mid-'90s, both of these Ebola types have wreaked havoc on Africa every few years. Most of these outbreaks have been contained to central Africa quite close to the equator in countries like the Democratic Republic of Congo, Gabon and Uganda. In March 2014, however, the first outbreak of Zaire Ebola was seen outside of this region in the western African country of Guinea, , quickly spreading to nearby countries (Liberia, Sierra Leone, Mali, Senegal and Nigeria) and even going beyond Africa into the United States and Spain. Scientists are guessing that if, in fact, the virus is linked to bats, changes in climate could be causing differences in migration patterns of these bats, leading to the spread of Ebola beyond its origin of central Africa.

The two other African Ebola varieties, Ivory Coast and Bundibugyo, have only been observed three times with fairly low fatality rates. And finally, the fifth family member, the Reston type, seems to be most fatal to nonhuman primates. This particular strain also found its way into the U.S. (it's actually named after a fairly tame suburb of Washington, D.C.) in 1989 when an Ebola outbreak erupted from a primate research facility. Workers at the facility were thoroughly screened for symptoms and signs of the virus. Thankfully, this turned out to be the one type of filovirus that doesn't worm its way into humans.

(The CDC has a map of Ebola outbreaks in Africa if you're interested.)

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