The NFL is a thresher of humans. Sunday, when the nastiest team in a nasty league visits U.S. Bank Stadium, “survive and advance” will adopt new meaning.

In recent weeks, the Normalized Felony League has featured one of its greatest players, Rob Gronkowski, smashing a prone opponent in the head well after the whistle, causing a concussion.

And Houston quarterback Tom Savage taking a blow to the head, twitching in what appeared to be convulsions, then returning to the field.

And a Washington Post report about former star running back Larry Johnson, who says he believes he is suffering from chronic traumatic encephalopathy (CTE) and feels suicidal.

And a game between the Bengals and Steelers that offered everything but bloody folding chairs.

And the continued winnowing of stars, as Philadelphia’s Carson Wentz was lost for the season, joining a list of injured players almost as impressive as the list of healthy ones.

Sunday, a lousy Cincinnati Bengals team will face one of the NFL’s best success stories. Which means that one or two cheap shots from a cheap team could ruin the Vikings’ season.

What’s most alarming about the carnage is that it is occurring as awareness of damaging effects of brain injuries continues to grow. Players don’t joke about concussions anymore. Now, they just don’t care, at least not while they’re playing. Education has not altered behavior.

I’ve been fascinated by NFL violence and our reaction to it for decades. Three brief conversations over the years have stuck with me.

Asked longtime NFL writer Mike Freeman why football is America’s most popular sport. “Violence,’’ he said. He didn’t elaborate, or need to.

Asked Vikings safety Harrison Smith, when he was a rookie known for launching himself headfirst into opponents, whether he worried about the health of his own brain, or body. “Nah,’’ he said, not rudely or angrily, but as if I had asked whether he would like fresh ground pepper on his salad.

Asked legendary Eden Prairie coach Mike Grant, Bud’s son, why we love the game. “There’s a sense when you all take the field,’’ he said, “that not everyone’s coming back whole.’’

Wentz joined Sam Bradford, Aaron Rodgers, Andrew Luck, Deshaun Watson, Josh McCown and Ryan Tannehill among quarterbacks who have missed significant time this season.

They join Richard Sherman, J.J. Watt, Dalvin Cook, Odell Beckham Jr., Brandon Marshall, Julian Edelman, Jason Peters, Joe Thomas, Marshal Yanda, Eric Berry, David Johnson and Darren Sproles among star-quality players who have been lost for the season.

Wednesday, Vikings coach Mike Zimmer said Bengals coach Marvin Lewis should keep his job. “He should be able to stay there as long as he wants,’’ Zimmer said.

Even if you want to overlook Lewis’ inability to win one playoff game in 15 years in Cincinnati, Zimmer would be wrong.

Lewis runs the dirtiest team in the NFL. Earlier this month, Steelers receiver JuJu Smith-Schuster blindsided Bengals linebacker Vontaze Burfict with a helmet-to-helmet hit, then stood over Burfict.

That was wrong, yet karmic. Burfict has been leveling cheap shots at opponents for years.

Which is why brutal and cheap hits damage the game, as well as brains, spines and joints. Those who believe in exchanging an eye for an eye will soon be blind.

The Bengals’ ruthlessness makes this one of the most concerning games on the Vikings’ schedule. The Vikings should win, but will they be able to avoid the kind of injury that could damage their chances in the playoffs?

Sunday, the Vikings can do themselves a favor by keeping their heads on a swivel, and they can do the NFL a favor by moving Lewis one game closer to unemployment.