Former San Francisco 49ers wide receiver and two-time Super Bowl champion Dwight Clark revealed on his blog on Sunday that he has ALS, a nervous system disease that weakens the muscles and cannot be cured.

In the post, Clark said he began feeling weakness in his left hand in 2015. After tests, he was diagnosed with ALS.

"While I’m still trying to wrap my head around the challenge I will face with this disease over the coming years, the only thing I know is that I’m going to fight like hell and live every day to the fullest," he wrote.

Clark said he's also lost strength in his other hand, abs, lower back and right leg. He said he is unable to walk or pickup anything over 30 pounds. Fortunately, he said the disease seems to be progressing slower than in most patients.

"I’ve been asked if playing football caused this. I don’t know for sure. But I certainly suspect it did. And I encourage the NFLPA and the NFL to continue working together in their efforts to make the game of football safer, especially as it relates to head trauma," he said.

Clark played for the 49ers from 1979 to 1987. He is best known for catching the winning touchdown pass from Joe Montana in the 1982 NFC Championship against the Dallas Cowboys, known as "The Catch." He was a general manager of the 49ers in 1998 and the Cleveland Browns from 1998 to 2002.

Read his full blog post here.