'I was mildly paying attention to it because since my playing days, I've constantly had pain in my neck, ' Clark wrote. "I was thinking it was related to some kind of nerve damage because it would just come and go".
"After months of tests and treatment, I got some bad news. I was diagnosed with amyotrophic lateral sclerosis", Clark wrote on his blog.
"What I do know is I have a huge battle in front of me and I'm grateful for the strength and unconditional love from my wife Kelly".
"Those words are still very hard for me to say", the former wide receiver wrote.
Clark is best known for teaming up with Joe Montana to make of the most famous plays in football history. I have ALS, also known as Lou Gehrig's disease. It gradually destroys the brains ability to communicate with muscles, typically causing death when the muscles needed for breathing become too weak.
Dwight Clark announced he has Lou Gehrig's disease and suspects playing football might have caused the illness. But I certainly suspect it did.
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Clark, whose No. 87 has been retired by the 49ers, also encouraged the NFL and the players' association to work together in making football safer.
"I can't run, play golf or walk any distances", he said.
He felt the first symptoms, weakness in his left hand, in late 2015. Fortunately, he said the disease seems to be progressing slower than in most patients. In 8 seasons for the 49ers, Clark totaled 506 catches for 6750 yards and 48 touchdowns in 134 career games.
In one of the most memorable plays in National Football League history, Clark caught a pass from Joe Montana in the very back of the end zone with under a minute left to play to give San Francisco a 28-27 win and a spot in Super Bowl XVI. The TD lifted San Francisco to a 28-27 victory over the Dallas Cowboys and into Super Bowl XVI, where the 49ers defeated the Cincinnati Bengals 26-21.
Clark made the announcement on Eddie DeBartolo's website, and the former 49ers owner has made it clear the DeBartolo family will be there to support the Clark family and help them fight.
San Francisco CEO Jed York said in a statement he was "deeply saddened" by Clark's diagnosis. "We hope the public will be cognizant of Dwight's desire for privacy".