San Francisco 49ers linebacker Chris Borland, center, during an NFL football game in Santa Clara, Calif. Borland announced that he will retire after just one season to protect himself from brain injuries. Marcio Jose Sanchez/AP hide caption

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Football rules, uniforms, helmets and protective gear have changed a lot over the years. Keystone-France/Gamma-Keystone via Getty Images hide caption

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Lt. Gen. Mark Milley speaks to reporters April 2 regarding the second shooting in five years on the Fort Hood Army post in Texas. Drew Anthony Smith/Getty Images hide caption

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Researchers have only recently been able to use brain scans to detect Alzheimer's risk factors in living people. iStockphoto hide caption

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Kansas City Chiefs linebacker Jovan Belcher's body has been exhumed more than a year after he killed his girlfriend and himself so that his brain can be examined for signs of a degenerative condition linked to repeated concussions. Seth Perlman/AP hide caption

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Dartmouth defenders sandwich a New Hampshire wide receiver during a game in Durham, N.H., in 2009. Josh Gibney/AP hide caption

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Hospital intensive care units save lives, but people there often suffer from delirium. Cal EMA / Flickr hide caption

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The cause of strokes in younger people remains largely a mystery. hide caption

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Dr. Ann McKee, professor of neurology and pathology of Boston University School of Medicine and co-director of the Veterans Affairs Center for the Study of Traumatic Encephalopathy, inspects a brain in the Bedford Veteran Medical Center last year. Stan Grossfeld/Boston Globe via Getty Images hide caption

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