E-cigarettes work by heating up a fluid that contains the drug nicotine, producing a vapor that users inhale. The devices are most popular among young adults, ages 18 to 24, a federal survey indicates. iStockphoto hide caption

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The large British study, begun in 1958, tracked the diet, habits and emotional and physical health of thousands of people from childhood through midlife. iStockphoto hide caption

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Maria Fabrizio for NPR

People who don't get enough sleep show higher levels of inflammation, say scientists who study colds. Smoking, chronic stress and lack of exercise can make you more susceptible to the viruses, too. Frederic Cirou/PhotoAlto/Corbis hide caption

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Detweiler was surprised to learn she wasn't eating enough to fuel her training regimen. As an athlete, doctors and nutritionists say, she needed more food variety and more calories — three snacks daily, as well as bigger meals. Courtesy of Nationwide Children's Hospital hide caption

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Ten-year-old Jake Herrera and his Los Angeles team run around the diamond as a warmup for baseball practice. Benjamin B. Morris for NPR hide caption

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Dr. Donald Brown inoculated Kelly Kent with the HPV vaccine in his Chicago office in the summer of 2006 — not long after the first version of the vaccine reached the market. Charles Rex Arbogast/AP hide caption

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Kids as young as 13 purchase e-cigarettes, or "vape pens," online, where independent sellers don't necessarily ask a buyer's age. Jenny Lei Bolario/Youth Radio hide caption

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Solid friendships can help buffer life's stress. iStockphoto hide caption

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Condom use has dropped among black youth, even as teens engage in less risky sexual behavior overall. Mike Segar/Reuters/Landov hide caption

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