Polio Vaccine May Be Preventing The End Of Polio

Photo taken Sun., Aug. 28, 2016. A health official inks a child finger to indicate she has been administered with a polio vaccine at a camp of people displaced by Islamist extremist in Maiduguri, Nigeria.

Health officials have long known that virus from the oral vaccine can contaminate water systems; they underestimated how big a problem this would be.

(Image credit: Sunday Alamba/AP)