Many Pakistanis, especially in rural areas, view vaccinations campaigns with suspicion as a Western plot to sterilize Muslims. In December, nine health officials working to immunize Pakistanis against polio were killed by militants opposed to the campaign. Pakistan is one of three countries in the world where polio is endemic.
Sindh province, the area hardest hit by the measles outbreak, has also been battered by repeated floods in recent years that have damaged hospitals and clinics.
Measles is an extremely infectious disease spread by coughing and sneezing or personal contact. It causes a fever, cough and a rash all over the body. Most people who contract the disease recover, but it can be fatal for malnourished children.
Complications from the disease can include blindness, an infection that causes brain swelling, dehydration and diarrhea, and pneumonia.
According to WHO, 139,300 people died of measles worldwide in 2010.