The cause of strokes in younger people remains largely a mystery. iStockphoto.com 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

itoggle caption Stan Grossfeld/Boston Globe via Getty Images

Sgt. Victor Medina suffered brain damage in 2009 when a roadside bomb exploded in Iraq. Blake Gordon/Aurora Photos hide caption

itoggle caption Blake Gordon/Aurora Photos