The Super Bowl is a beloved American tradition, with a heavy cash flow that leads back to the National Football League -- ad spots this year went for as much as $5 million and spectators were willing to pay $4,600 for game day tickets at secondary merchants.

As the NFL’s coffers have grown, the league has increasingly been able to deflect some of the concerns over the impact the game has had on the health of players by expanding its influence on research.

Yet, the disease chronic traumatic encephalopathy (CTE) has been diagnosed in 87 out of 91 former NFL players as of 2015 and continues to trigger debates on the fate of the game's future ranging from peewee leagues to the pros.

Even after more than 5,000 ex-players sued the NFL, saying it hid the dangers of head injuries, the league has so far denied claims the sport is dangerous.

Just two days ago, league Commissioner Roger Goodell doubled down on the game's safety and said he'd even encourage his young son to play football.

Every case of diagnosed CTE has had one thing in common: a history of repetitive hits to the head. Robert Stern, director of clinical research for Boston University's CTE center

Doctors have known about CTE since the 1920s, when it was primarily associated with boxers and referred to as "dementia pugulistica," according to Robert Stern, director of clinical research for Boston University's Chronic Traumatic Encephalopathy Center.

The link between CTE and football didn't pick up steam until 2005 when forensic pathologist Bennet Omalu (recently played by Will Smith in the film "Concussion") published research following his diagnosis that retired NFL star "Iron Mike" Webster had CTE.

"It took the diagnosis in a professional football player, after death, to make this more of a salient topic, even though we’ve known about it in boxers for so long," said Stern, who has been notably critical of the league.

CTE is a degenerative disease that’s similar in ways to Alzheimer's disease, but also different from other brain diseases in that it's neither a brain injury nor cumulative.

"The thing with CTE that’s different with Alzheimer's and other degenerative diseases is that we know the thing that gets it started -- exposure to repetitive head impacts," Stern said. "CTE is a brain disease that gets started early in life."

Symptoms of CTE include cognitive impairment like memory and multitasking; mood problems like depression and apathy; behavior changes like aggression and impulse control; and, most rarely, motor problems like body tremors or difficulty making facial expressions.

CTE manifests itself in different ways depending on the person, and can be present for decades before symptoms are visible.

"You have younger guys like Junior Seau or Dave Duerson who were very young and had years of suicidal thoughts, and you have other people like Frank Gifford, who had dementia," Stern said, who has found CTE in people as young as 18.

Last week, former NFL quarterbacks Ken Stabler and Earl Morrall were added to the growing list of players who had CTE, which can only be diagnosed after death.

"We’re still really at the beginning of our understanding about this disease," Stern said. "But every case of diagnosed CTE has had one thing in common: a history of repetitive hits to the head."

As the list of former NFL players with CTE grows, here's a look at some of the most prominent cases:

Tyler Sash, 27 Jason O. Watson/Getty Images Defensive back and Super Bowl winner Tyler Sash died in 2015 at the age of 27 due to a painkiller overdose. After his family donated Sash's brain, Boston University researchers said he had CTE that "advanced to a stage rarely seen in someone his age."

Mike Webster, 50 George Gojkovic/Getty Images Mike Webster's 2002 autopsy has been called the examination that "changed football." Neuropathologist Bennet Omalu's resulting report that linked playing football to CTE was the focus of a Frontline investigation and the feature film "Concussion" -- and sparked countless conversations about the safety of the game.

Earl Morrall, 79 Kidwiler Collection/Getty Images When quarterback Earl Morrall died in 2014 at age 79 due to issues with Parkinson's, he had the most severe state of CTE, his family later confirmed

Dave Duerson, 50 Kidwiler Collection/Getty Images Former Chicago Bears defensive back Dave Duerson shot himself in the chest at age 50 after reportedly suffering years of cognitive and motor issues. He left behind a suicide note that read, “Please, see that my brain is given to the N.F.L.’s brain bank.” Duerson was later found to have CTE.

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