Football players, boxers, or anyone who gets repeated blows to the head can get this condition, which kills brain cells. A single concussion isn’t likely to cause it. Symptoms often don’t show up for years. At first they include problems with mood, behavior, and impulse control. Memory loss, problems making rational decisions, and dementia come later. Doctors can’t diagnose it until after death, when they can look at your brain. There’s no treatment for the disease, only its symptoms.