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

The Ethiopian government has set up about a dozen vaccination booths along its thousand-mile border with Somalia. Jason Beaubien/NPR hide caption

itoggle caption Jason Beaubien/NPR

A young girl stands in the doorway of a home in Uttar Pradesh, India. The markings on the door show that a polio immunization team vaccinated children in the home. Alyce Henson/Courtesy of Rotary International hide caption

itoggle caption Alyce Henson/Courtesy of Rotary International

Syrian opposition fighters sit on the front line in the city of Deir Ezzor on Oct. 13. Ongoing violence has ravaged the city since March 2011. AFP/Getty Images hide caption

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Avez, 2, is held by his mother, as he receives the polio vaccine in the Khyber Tribal Region in northwest Pakistan. Daniel Berehulak/Getty Images hide caption

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In early August, Israel launched a mass campaign to vaccinate children against polio, including this little girl at a clinic in Rahat. David Buimovitch/AFP/Getty Images hide caption

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Ali Maow Maalin said he avoided getting the smallpox vaccine as a young man because he was afraid of needles. He didn't want others to make the same mistake with polio. Courtesy of the World Health Organization hide caption

itoggle caption Courtesy of the World Health Organization