A health worker vaccinates a child during a polio campaign in Bannu, Pakistan, June 25. The Taliban threaten to kill vaccinators and parents who immunize their kids. A Majeed/AFP/Getty Images hide caption

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Usman (right), 7 months, and Abdullah (left), 18 months, are held by their mothers while they wait to receive the polio vaccine at the Jalozai refugee camp near Peshawar, Pakistan. Daniel Berehulak/Getty Images hide caption

itoggle caption Daniel Berehulak/Getty Images

During nationwide polio campaigns, hundreds of thousands of health workers go door to door, giving children two drops of the polio vaccine. Anadolu Agency/Getty Images hide caption

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A health worker gives a child the polio vaccine in Bannu, Pakistan, June 25. More than a quarter-million children in Taliban-controlled areas are likely to miss their immunizations. A. Majeed/AFP/Getty Images hide caption

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By ensuring vaccines are invented and distributed, Bill Gates says, his foundation is dramatically reducing the number of childhood deaths in poor countries. Marie McGrory/NPR hide caption

itoggle caption Marie McGrory/NPR

A child receives a polio vaccine Sunday in Kano, Nigeria. The country is the primary source of the virus in Africa but appears to be making progress against the disease; the current outbreak in Cameroon that has spread to Equatorial Guinea came by way of Chad, not Nigeria. Sunday Alamba/AP hide caption

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A boy waits to get vaccinated at an anti-polio campaign in Moradabad, India. Michaeleen Doucleff/NPR hide caption

itoggle caption Michaeleen Doucleff/NPR

Eight-year-old Manish, who caught polio years ago, learns to walk with leg braces at a rehabilitation center in New Delhi on Thursday. Manish Swarup/AP hide caption

itoggle caption Manish Swarup/AP

Sophia Jarvis, 4, of Berkeley, Calif., is one of the few children diagnosed with the polio-like disease, which left her arm paralyzed. She attended a press conference Monday at Stanford University with her dad, Jeff. Martha Mendoza/AP hide caption

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Young boys suspected of having Guinea worm infections wait for the parasite to emerge through the skin at a clinic in South Sudan. Louise Gubb/The Carter Center hide caption

itoggle caption Louise Gubb/The Carter Center

A polio worker vaccinates a child in Khyber-Pakhtunkhwa province, Pakistan, in October. Arshad Arbab/EPA/Landov hide caption

itoggle caption Arshad Arbab/EPA/Landov

Syrian boys line up to get the polio vaccine at a refugee camp in Sidon, Lebanon, on Nov. 7. The Lebanese government plans to vaccinate all kids under age 5 for the virus, including Syrian refugees. Mohammad Zaatari/AP hide caption

itoggle caption Mohammad Zaatari/AP

A doctor vaccinates a child against polio at a health clinic in Damascus, Syria, on Nov. 6. To stop the disease from spreading beyond Syria, health officials plan to vaccinate 20 million children in the region. Youssef Badawi/EPA /LANDOV hide caption

itoggle caption Youssef Badawi/EPA /LANDOV