The Ebola outbreak in West Africa is the most explosive in history. One reason the virus spread so fast is that West Africa was blindsided. Ebola had never erupted in people anywhere close to West Africa before.
The type of Ebola causing the outbreak — called Zaire — is the deadliest strain. Until this year, it had been seen only in Central Africa, about 2,500 miles away. That's about the distance between Boston and San Francisco.
So how did it spread across this giant swath of land without anybody noticing?