Wild poliovirus cases nearly doubled from 2012 to 2013, in part because the virus spread from the remaining 3 endemic counties to neighboring nations in Africa and the Middle East.

A recent study from the Centers for Disease Control and Prevention and the World Health Organization (WHO) showed that 8 countries reported 416 wild poliovirus cases in 2013, an 86% increase from the 223 cases that 5 countries reported in 2012. As of May 20, 2014, 82 cases were reported from 8 countries, compared with 34 cases from 3 countries during the same period last year (Moturi EK et al. MMWR Morb Mortal Wkly Rep. 2014;63[21]:468-472).