http://www.sciencemag.org/news/2018/07/polio-outbreaks-congo-threaten-global-eradication

The weakened virus in polio vaccine drops can, on rare occasions, regain virulence, sparking outbreaks.

Overshadowed by the Ebola outbreak in the Democratic Republic of the Congo (DRC), another frightening virus is on the loose in that vast, chaotic country: polio. Public health experts have worked for months to stamp out the virus, but it keeps spreading. It has already paralyzed 29 children, and on 21 June a case was reported on the border with Uganda, far outside the known outbreak zone, heightening fears that the virus will sweep across Africa. The DRC is “absolutely” the most worrisome polio outbreak today, says Michel Zaffran, who heads the Global Polio Eradication Initiative (GPEI) at the World Health Organization (WHO) in Geneva, Switzerland.