A recent study of over 3,000 retired professional football players found that they were 4 times as likely to die from a neurodegenerative disease such as Alzheimer's disease or amytrophic lateral sclerosis (ALS) than the general population. In addition, "speed" positions such as quarterbacks, running backs, or line receivers were 3 times more likely to develop a neurodegenerative disease than "non-speed" positions such as linemen.
Multiple sports-related concussions increase the risk of developing chronic traumatic encephalopathy (CTE), a neurodegenerative disease that is very similar to Alzheimer's disease and ALS. Although it's a separate diagnosis, CTE may be a trigger for developing these other diseases.
Lehman EJ, Hein MJ, Baron SL, & Gersic CM. Neurodegenerative causes of death among retired National Football League players. Neurology. (September 2012)