My husband, Rob Kelly, is a retired N.F.L. player. After five seasons as a safety beginning in the late 1990s, four with the New Orleans Saints and one with the New England Patriots, he sustained an injury to a nerve between his neck and shoulder during training camp that ended his career. By the time he retired in 2002 at 28, he had been playing tackle football for about two decades.

Rob had no idea, however, that all those years of playing would have such serious consequences. Safeties are the last line of defense and among the hardest hitters in the game. One tackle he attempted while playing for the Saints was so damaging, he doesn’t remember the rest of the game. He got up, ran off the field and tried to go back in — as an offensive player. He knows this only because people told him the next day.

Professional football is a brutal sport, he knew that. But he loved it anyway. And he accepted the risks of bruises and broken bones. What he didn’t know was that along with a battered body can come a battered mind.

For decades, it was not well understood that football can permanently harm the brain. Otherwise, many parents would most likely not have signed their boys up to play. But this reality was obscured by the N.F.L.’s top medical experts, who for years had denied any link between the sport and long-term degenerative brain diseases like chronic traumatic encephalopathy.