WASHINGTON — Autumn, which is bearing down upon us like a menacing linebacker, is, as John Keats said, a season of mists and mellow fruitfulness and chronic traumatic encephalopathy (CTE). Actually, Keats, a romantic, did not mention that last part. He died before the birth of the subject of a waning American romance, football. This sport will never die but it will never again be, as it was until recently, the subject of uncomplicated national enthusiasm.

CTE is a degenerative brain disease confirmable only after death, and often caused by repeated blows to the head that knock the brain against the skull. The cumulative impacts of hundreds of supposedly minor blows can have the cumulative effect of many concussions. The New York Times recently reported Stanford researchers’ data showing “that one college offensive lineman sustained 62 of these hits in a single game. Each one came with an average force on the player’s head equivalent to what you would see if he had driven his car into a brick wall at 30 mph.”

Boston University researchers found CTE in 110 of 111 brains of deceased NFL players. In 53 other brains from college players, 48 had CTE. There was significant selection bias: Many of the brains came from families who had noticed CTE symptoms, including mood disorders and dementia. A BU researcher says, however, that a 10-year NFL linebacker could receive more than 15,000 sub-concussive blows.

Football’s kinetic energy — a function of the masses and velocities of the hurtling bodies — has increased dramatically in 50 years. On Alabama’s undefeated 1966 team, only 21 percent of the players weighed more than 200 pounds. The heaviest weighed 223; the linemen averaged 194. The quarterback, who weighed 177, was Ken Stabler, who went on to a Hall of Fame NFL career — and to “moderately severe” CTE before death from cancer. Today, many high school teams are much beefier than the 1966 Crimson Tide. Of the 114 members of Alabama’s 2016 squad, just 25 weighed less than 200 and 20 weighed more than 300. In 1980, only three NFL players weighed 300 or more pounds. Last season, 390 weighed 300 pounds or more, and six topped 350.

Players love football, and a small minority will have lucrative post-college NFL careers. Many will make increasingly informed choices to accept the risk-reward calculus. But because today’s risk-averse middle-class parents put crash helmets on their tykes riding tricycles, football participation will skew to the uninformed and economically desperate. But will informed spectators become queasy about deriving pleasure from an entertainment with such human costs?

No. They will say: Players know the risks that they, unlike the baited bears, voluntarily embrace, just as smokers do. Notice, however, that smoking, which is increasingly a choice of those least receptive to public health information, is banned in all NFL stadiums and is severely discouraged on all college campuses, including those that are football factories. And football fans will say: Better equipment will solve the problem of body parts, particularly the one in the skull’s brain pan, that are unsuited to the game.

Perhaps evolving standards of decency will reduce football to a marginalized spectacle, like boxing. But the UFC’s (Ultimate Fighting Championship’s) burgeoning popularity is (redundant) evidence that “evolving” is not a synonym for “improving.”

Besides, as disturbing scientific evidence accumulates, NFL franchise values soar (Forbes says the most valuable is the Dallas Cowboys at $4.2 billion and the least valuable is the $1.5 billion Buffalo Bills) and annual revenues reach $14 billion. The league distributes $244 million to each team — $77 million more than each team’s salary cap. Local revenues are gravy. The appendage of higher education that is called college football also is a big business: The Southeastern Conference’s cable television channel is valued at almost $5 billion. Universities, who find and develop the NFL’s players, pay their head coaches well for performing this public service: Twenty head coaches make more than $4 million a year. Michigan’s Jim Harbaugh earns $9 million.

It has been said (by Thomas Babington Macaulay) that the Puritans banned bear baiting — unleashing fierce dogs on a bear chained in a pit — not because it gave pain to bears but because it gave pleasure to Puritans. But whatever the Puritans’ motives, they understood that there are degrading enjoyments. Football is becoming one, even though Michigan’s $9 million coach has called it “the last bastion of hope in America for toughness in men.” That thought must amuse the Marines patrolling Afghanistan’s Helmand Province.