There's no better deal than getting polio cases down to zero, philanthropist Bill Gates says. Marie McGrory/NPR hide caption

itoggle caption Marie McGrory/NPR

A health worker in Pakistan marks a child's finger with ink after giving him the polio vaccine. Jackie Northam/NPR hide caption

itoggle caption Jackie Northam/NPR

Ado Ibrahim carries his son Aminu through a village in northern Nigeria. Aminu, 4, was paralyzed by polio in August. David Gilkey/NPR hide caption

itoggle caption David Gilkey/NPR

Health workers transport the polio vaccine by donkey in southeastern Pakistan. Asad Zaidi/Courtesy of UNICEF Pakistan. hide caption

itoggle caption Asad Zaidi/Courtesy of UNICEF Pakistan.

An Indian child receives the oral polio vaccine. Twice a year, an army of 2 million volunteers fans out across India to administer the vaccine. India has not reported a single case of polio in more than a year-and-a-half. Julie McCarthy/NPR hide caption

itoggle caption Julie McCarthy/NPR

Children in a Lahore slum after heavy rains. The slum has a large population of Pashtuns who came from Pakistan's lawless tribal regions; many carry the polio virus with them. Jackie Northam/NPR hide caption

itoggle caption Jackie Northam/NPR

Sahya Idriss, a service provider at the health clinic in Minjibir, carries a vial of the polio vaccine. David Gilkey/NPR hide caption

itoggle caption David Gilkey/NPR

During the peak of the polio epidemic in the U.S., some hospital wards even had large, room-like iron lungs where multiple children lived. Courtesy of Boston Children's Hospital Archive hide caption

itoggle caption Courtesy of Boston Children's Hospital Archive