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

itoggle caption Mohammed Huwais/AFP/Getty Images

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.

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