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

Chasing Down Polio
To wipe out polio from earth, health workers must stop it in Nigeria, Pakistan and Afghanistan.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
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
NPR's Jackie Northam travels through the urban slums of Lahore, Pakistan, with Omer Feroze, a "social mobilizer," who works on polio vaccine campaigns. NPR hide caption
A Yemeni child receives a polio vaccine in the capital city of Sanaa. The Yemen government launched an immunization campaign last month in response to the polio outbreak in neighboring Somalia. Mohammed Huwais/AFP/Getty Images hide caption
Health workers transport the polio vaccine by donkey in southeastern Pakistan. Asad Zaidi/Courtesy of UNICEF Pakistan. hide caption
Ado Ibrahim carries his son Aminu through a village in northern Nigeria. Aminu was paralyzed by polio in August. David Gilkey/NPR hide caption
Sahya Idriss, a service provider at the health clinic in Minjibir, carries a vial of the polio vaccine. David Gilkey/NPR hide caption