A rare and serious reaction to the high altitude of Denver after a game with the Broncos cut short the 2007 season for the Pittsburgh Steelers free safety and eventually led to the removal of his spleen and gall bladder.
So Clark's return to the Steelers this season was an accomplishment made more remarkable by his play - he's the No. 2 tackler on the league's top-ranked defense and on his way to an unlikely matchup with the Arizona Cardinals in the Super Bowl.
Clark also delivered the defining blow in the victory over the Baltimore Ravens that sent the Steelers on to the Super Bowl, a crunching hit in the AFC title game that put running back Willis McGahee in the hospital.
“I just had an opportunity to make a play for the team,” said Clark, so woozy himself after the blow that he didn't recall McGahee being taken off on a stretcher. “I remember the steps leading up to the hit, but I don't remember it. I close my eyes anyway when I hit.”
Clark did remember “maroon,” “bell” and “car,” the three words he was required by the Steelers' medical staff to recall in order to leave Heinz Field on Sunday night.
McGahee, who was released from the hospital the next day and is expected to make a full recovery, didn't see Clark coming with shoulder lowered. Neither did the New England Patriots' Wes Welker, whom Clark laid out with a similar blow in November.
Those are incredible hits for a player generously listed as 5-foot-11 and 205 pounds, a player who started at LSU for three seasons but went undrafted, was cut by the New York Giants after two years and wasn't re-signed by the Washington Redskins after starting on their 2005 playoff team.
“I've come a long way,” Clark said. “It's an awesome feeling. I really wanted to play the Giants to tell you the truth, but I'd play the Duquesne basketball team in Tampa if they'd go. I'm just excited. Seven years [in the NFL] and I had never played in a championship game, so to get this opportunity to play in the Super Bowl is truly a blessing. You talk about the undrafted part. I think about the almost dying part.”
That brush with death actually dates to 2005, when the Redskins played the Broncos in Denver.
Clark led the team in tackles that day but five days later fell ill with what was diagnosed as a bruised spleen.
When the Steelers played in Denver two years later, the effect on Clark's spleen was more immediate. Clark played but was hospitalized afterward with intense, inexplicable pain in his left side.
Clark returned to Pittsburgh, and the club held him out of practices and games as doctors searched for the causes of what happened in Denver and for the high fever and constant pain he was suffering from.
About a month later, the doctors discovered an infection in his spleen, and Clark was rushed to a hospital for a splenectomy. His gall bladder was removed shortly thereafter. Clark lost 30 pounds during his ordeal and recovery before he rejoined his teammates for workouts last spring.
But now he's in the Super Bowl, and he will be in the starting lineup - something his partner at safety on the Redskins, Sean Taylor, never managed before he was murdered in 2007.
“Sean's always going to be in my mind,” said Clark, who was fined $5,000 by the NFL this season for etching tiny No. 21s in his eye black in Taylor's memory. “He was special to me. I think to myself a lot, 'How would he play?' ”
As physical as Clark. That's how.