A nurse in Hyderabad, India, gives a vaccine to a child. The immunization will protect against diphtheria, pertussis, tetanus and other diseases. Noah Seelam/AFP/Getty Images hide caption

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A child is vaccinated in Makadara Health Clinic, Nairobi, Kenya. Greg Warner/NPR hide caption

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The Ebola vaccine from a trial in Guinea needs to be kept at a temperature of minus 60 degrees Celsius, the World Health Organization says. Storage devices use jet fuel to keep the right temperature for up to five days in the field. Sean Hawkey/Sean Hawkey hide caption

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HPV vaccine prevents cervical cancer and other cancers by fending off the virus that causes them. But it's been a tough sell with doctors and parents. Joe Raedle/Getty Images hide caption

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The varicella zoster virus causes chickenpox in children, then lurks in the body for years and can cause painful shingles later in life. James Cavallini/Science Source hide caption

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A family receives treatment for cholera at a clinic run by Doctors Without Borders in Port-au-Prince, Haiti, in October 2011, a year after the overwhelming outbreak began. Ramon Espinosa/AP hide caption

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Most of the people who got measles in last year's outbreaks hadn't been vaccinated with the MMR vaccine. Photo illustration by Justin Sullivan/Getty Images hide caption

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Parents and teachers who oppose efforts to end the personal belief exemption on vaccinations rally in Los Angeles on April 14. Irfan Khan/LA Times via Getty Images hide caption

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Parents rally against SB 277, a California measure requiring schoolchildren to get vaccinated, outside the Capitol in Sacramento, Calif. Rich Pedroncelli/AP hide caption

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