FILE - in this file photo taken Tuesday, Nov. 26, 2013, Pakistani Sonya Javed, 20, right, holds her son Shahzeb, 1, to receive a polio vaccine by a health worker at her home in Islamabad, Pakistan. The World Health Organization said the northwestern Pakistani city of Peshawar has become the largest poliovirus reservoir in the world. (AP Photo/Muhammed Muheisen, File)