A worker for a polio prevention program was shot to death in Pakistan's Khyber Agency, officials said. Sources told Pakistan's Dawn News the gunmen left behind pamphlets warning that other health workers faced a similar fate if they worked for the vaccination program or any other non-governmental organization. Although there was no immediate claim of responsibility for the slaying, polio workers have been frequently accused of being spies or part of a plot to sterilize Muslims, Voice of America said. The World Health Organization has blamed militant violence for a surge in polio cases from 58 cases in 2012 to 72 this year.