It's a condition once known as being "punch drunk" because it affected boxers who suffered multiple blows to the head, but it is a growing occupational hazard for the hard hitting sport of football because players are bigger, faster and more powerful than ever.
Chronic traumatic encephalopathy (CTE), a degenerative brain disease, has been linked to repeated head trauma and to former pro football player Junior Seau, the 20-season veteran NFL linebacker who committed suicide last year.
Seau is the latest football player to commit suicide and later be diagnosed with CTE. Former NFL veterans Dave Duerson, Terry Long and Andre Waters all shot themselves to death and were later found to have the disease.
Earlier this year Kansas City Chiefs linebacker Jovan Belcher killed his girlfriend and then himself. It's not known whether his brain is being studied for CTE.
Two recent studies – one released by the National Institutes of Health Thursday -- have discovered abnormalities associated with the condition in the brains of former pro football players like Seau.
Of the 34 former NFL players who have died and donated their brains to research, the percentage of them who have pathologically confirmed CTE is staggering – over 90 percent, a 2009 University of Michigan report found.
"Despite improvements in technology and equipment and modifications to rules in the game on both the pro and amateur level, there's just a rougher style of play now than in the past," said Dr. Jaime Levine, the medical director of brain injury rehabilitation with the Rusk Institute in New York.
Levine noted that pro football players are a lot bigger than they used to be. According to ESPN statistics, from 1979 to 2011, the typical top-five offensive tackle enlarged from an average of 6-foot-4, 264 pounds to 6-foot-6, 314 pounds. From 1979 to 2011, NFL-bound centers grew from an average 6-foot-3, 242 pounds to 6-foot-4, 304 pounds. In the same period, guards enlarged from an average 6-foot-3, 250 pounds to 6-4, 317 pounds.
"Size and physical conditioning techniques in sports at all levels have evolved to create an intense athlete," she said. "They're able to create more force, power and speed than ever before and that leads to harder hits and a greater number of hits."
Harvard neurologist, Dr. Marie Pasinski, said part of the problem is that brain trauma almost certainly begins years before an athlete enters the professional arena. According to the American Association of Neurological Surgeons, between 4 percent and 20 percent of college and high school football players sustain at least one brain injury during the course of a single season.
Pasinski said she views CTE as a kind of overuse syndrome, the result of repeated injury rather than one catastrophic event.
"Sports have become more competitive over time and kids now seem to practice a lot more than in previous generations. Think of the player who works out with the team 12 months of the year," Pasinski said.
Damage from this sort of mild, "subclinical" head traumas add up over time. Shots to the head that don't necessarily cause obvious symptoms lead coaches and trainers to assume -- mistakenly – that a player is fine to get back in the game. And sometimes athletes keep quiet about this type of injury because they want to keep playing.
"There is still a pervasive belief that only a concussion serious enough to knock the athlete out will do damage, but that's not the case," Pasinski said. "Any blow to the head that leaves a person slightly dazed or not quite right may cause harm to the brain."