A baby receives a polio vaccine at the Medina Maternal Child Health center in Mogadishu, Somalia. The country has one of the lowest immunization rates in the world. Ben Curtis/AP hide caption

itoggle caption Ben Curtis/AP

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 baby receives a polio vaccine at the Medina Maternal Child Health center in Mogadishu, Somalia, on Wednesday. Somalia has one of the lowest immunization rates in the world. Ben Curtis/AP hide caption

itoggle caption Ben Curtis/AP

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

A Nigerian health commissioner Dr. Sani Malam vaccinates a child for polio during a national immunization drive in Bauchi, Nigeria, last week. Deji Yake/EPA /Landov hide caption

itoggle caption Deji Yake/EPA /Landov

A child is inoculated with the polio vaccine at a traffic checkpoint just outside Pakistan's capital, Islamabad. Roadside vaccinations help health workers reach children in mobile populations. Jackie Northam/NPR hide caption

itoggle caption Jackie Northam/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