The H1N1 swine flu virus kills some people, while others don't get very sick at all. A genetic variation offers one clue. Centre For Infections/Health Pro/Science Photo Library/Getty Images hide caption

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Measles is highly contagious, and it produces fever and rash in susceptible people who become infected. Hazel Appleton/Health Protection Agency Centre/Science Source hide caption

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Students use tablets in a classroom in Mae Chan, a remote town in Thailand's northern province. Christophe Archambault/AFP/Getty Images hide caption

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Sara Martín reads bedtime stories with her children. When the kids were younger, she says, staying up to date on their frequent immunizations was tough, because of cost and transportation issues. Lauren M. Whaley/CHCF Center for Health Reporting hide caption

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Measles vaccine isn't a part of most workplaces. Eric Risberg/AP hide caption

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California is one of 20 states that allow parents to opt out of vaccination requirements for reasons of "personal belief." About 10 percent of students in the Santa Monica-Malibu school district are not immunized. Justin Sullivan/Getty Images hide caption

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Dr. Eric Ball examines a healthy 5-day-old patient in his office in Ladera Ranch, Calif. Ball and colleagues decided this week to take only patients whose parents follow the recommended vaccine schedule. Courtesy of Eric Ball hide caption

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Jackie Carnegie immunizes Mabel Haywood in a Colorado Health Department immunization van in 1972. Shots for measles and other infectious diseases were offered. Ira Gay Sealy/Denver Post Archive/Getty Images hide caption

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Juniper Russo walks her dogs with her daughter Vivian (left). Courtesy of Juniper Russo hide caption

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Dr. Ben Carson speaks to guests at the Iowa Freedom Summit on January 24, 2015 in Des Moines, Iowa. Scott Olson/Getty Images hide caption

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Writer Roald Dahl and his wife, actress Patricia Neal, with two of their children, Theo and Chantel Sophia "Tessa." The photo was taken a few years after oldest daughter, Olivia, died of measles. Hulton Archive/Getty Images hide caption

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A nurse administers an experimental Ebola vaccine Monday at Redemption Hospital in Monrovia, Liberia. Researchers aim to give shots to 27,000 people during the large trial. John Moore/Getty Images hide caption

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Rhett Krawitt, 6, outside his school in Tiburon, Calif. Seven percent of the children in his school are not vaccinated. Courtesy of Carl Krawitt hide caption

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Bruno Mbango Enyaka gets his flu shot at a community health center in Portland, Maine, on Jan. 7. Gabe Souza/Press Herald via Getty Images hide caption

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