A child is immunized against polio at the health clinic in a farming village in northern Nigeria. The procedure involves pinching two drops of the vaccine into the child's mouth. For full protection, the child needs three doses, spaced out over time. David Gilkey/NPR hide caption

itoggle caption David Gilkey/NPR

An Indian boy receives a polio vaccination from an Indian health worker in Amritsar last year. Narinder Nanu/AFP/Getty Images hide caption

itoggle caption Narinder Nanu/AFP/Getty Images