Gunmen shot and killed a police officer guarding a polio immunization team on Wednesday, the latest attack on efforts to protect children from the crippling and sometimes deadly disease. The police constable was on duty with a polio team in the remote Buner district in KPK when unknown gunmen opened fire, police chief Irshad Khan Yousafzai said. "He was shot in the head and shoulder and died immediately," he said, adding that officers had been assigned to polio teams in the district after a rise in threats against workers.