Kansas City Chiefs linebacker Jovan Belcher shot and killed his girlfriend before killing himself two years ago. Pathology reports show he likely had chronic traumatic encephalopathy, or CTE, doctors say. Scientists believe repeated head trauma can cause CTE, a progressive degenerative disease of the brain. Symptoms include depression, aggression, and disorientation, but so far scientists can only definitively diagnose it after death. Here are a few of the former athletes who have been diagnosed with CTE.