Blows to the head during contact sports like football, boxing, and soccer can cause injury to the brain, called traumatic brain injury. Studies of American football players have identified a serious consequence of repeated traumatic brain injuries. In a condition called chronic traumatic encephalopathy, or CTE, tangles of a protein called tau build up in the brain after repeated head impacts. The resulting brain damage is similar to that seen in Alzheimer’s disease. CTE can lead to dementia and eventually death.