"Professional football players do not sustain frequent repetitive blows to the brain on a regular basis." So concluded the National Football League in a December 2005 scientific paper on concussions in America's most popular sport. That judgment, implausible even to a casual fan, also contradicted the opinion of a growing cadre of neuroscientists who worked in vain to convince the NFL that it was facing a deadly new scourge: A chronic brain disease that was driving an alarming number of players -- including some of the all-time greats -- to madness.