Dr. Ann McKee has examined brain tissue from more former NFL players than any other scientist. She studies chronic traumatic encephalopahy or CTE. The disease destroys the integrity of brain cells over time and leads to problems with memory, agitation, and anger. It is believed to be common among older, former professional football players who suffered repetitive blows to their heads. In the video from FRONTLINE: League of Denial on the next page, Dr. McKee is surprised to discover the disease in young players.