A gunman recently shot and killed a police officer for the crime of guarding polio-eradication workers who were attempting to help young children by giving them a vaccine. My money's on the Taliban:

There was no immediate claim of responsibility for the attack, but the Pakistani Taliban have repeatedly targeted health workers on anti-polio drives, accusing them of being spies. The violence has seriously hindered the immunization campaign in Pakistan, which is one of three countries in the world where polio remains endemic.

The resistance to polio vaccination by Taliban militants stiffened after the raid in May 2011 on Osama bin Laden’s compound in northern Pakistan, as revelations surfaced that the C.I.A. had used a Pakistani doctor, Shakil Afridi, to run a vaccination campaign to help in efforts to locate Bin Laden.

Since then, Taliban insurgents have increased their attacks on the vaccination work and issued religious edicts that accuse the United States of using it to gather intelligence. Hard-line religious extremists also claim that the vaccinations aim to sterilize Muslims.