On Sunday night, former 49ers wide receiver Dwight Clark, perhaps best known for “The Catch,” sent out a somber note on social media announcing he has been diagnosed with ALS.

ALS, or amyotrophic lateral sclerosis, is a degenerative nerve disease that affects “nerve cells in the brain and spinal cord.” It’s also known as “Lou Gehrig’s disease.”

According to a letter he penned on the DeBartolo Holdings website, Clark first became concerned when he felt “weakness in my left hand” back in September of 2015. Clark underwent “months of tests and treatment” before being diagnosed with ALS.

“In addition to losing strength in my left hand -- which makes opening a pack of sugar or buttoning my shirt impossible -- I have now experienced weakness in my right hand, abs, lower back and right leg,” Clark said. “I can’t run, play golf or walk any distances. Picking up anything over 30 pounds is a chore.”

And he’s worried football may have caused him to develop the disease.

“I’ve been asked if playing football caused this. I don’t know for sure. But I certainly suspect it did,” Clark wrote. “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.”

Clark, a native of Kinston, North Carolina, went to Clemson and was drafted by the 49ers in the 10th round of the 1979 NFL Draft. He would play eight seasons for the 49ers, finishing with 506 catches for 6,750 yards in his NFL career.

Jed York released the following statement after Clark’s announcement.

None was bigger than “The Catch,” a 6-yard touchdown reception in the back of the end zone to propel San Francisco over the Cowboys and into Super Bowl XVI.

In his career, Clark won two Super Bowl titles and led the league in receptions once.

Here is the full text of his letter that he posted to the public: