No one knows how many children have been affected by the spread of Boko Haram across Nigeria and neighboring countries. This photo shows the school uniforms of the girls kidnapped from a school a year ago. Glenna Gordon/Glenna Gordon for NPR hide caption

itoggle caption Glenna Gordon/Glenna Gordon for NPR

On the outskirts of Islamabad, a Pakistani health worker vaccinates an Afghan refugee against polio. Muhammed Muheisen/AP hide caption

itoggle caption Muhammed Muheisen/AP

A child receives a polio vaccine Sunday in Kano, Nigeria. The country is the primary source of the virus in Africa but appears to be making progress against the disease; the current outbreak in Cameroon that has spread to Equatorial Guinea came by way of Chad, not Nigeria. Sunday Alamba/AP hide caption

itoggle caption Sunday Alamba/AP

Sylvestre Mebam (right) runs an unauthorized health clinic in Cameroon's largest city of Douala. He and his assistant treat about 10 to 15 people each day. Andres Caballero/NPR hide caption

itoggle caption Andres Caballero/NPR