Former San Francisco 49ers WR Dwight Clark announced he has ALS

SANTA CLARA, Calif. (KGO) -- Former San Francisco 49ers wide receiver Dwight Clark took to Twitter Sunday to announce he has ALS and included a link to a letter he wrote. Click here to see how his former teammates are reacting.

In a letter, Clark wrote:

"In September of 2015, I started feeling weakness in my left hand. I was mildly paying attention to it because since my playing days, I've constantly had pain in my neck. 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.

I have ALS, also known as Lou Gehrig's disease. Those words are still very hard for me to say.

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.

There is no test that will positively diagnose you with ALS. You have to eliminate the possibility of all other

diseases and disorders and then wait to see what additional symptoms you develop. I visited six neurologists

and three ALS specialists. I also was treated for a B12 deficiency, which sometimes can mirror the symptoms of this debilitating disease.

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. I can't run, play golf or walk any distances. Picking up anything over 30 pounds is a chore. The one piece of good news is that the disease seems to be progressing more slowly than in some 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.

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. She has been my rock. She keeps thinking positive and convinces me each day that we can beat this, as does my daughter Casey and my son Mac. My brother Jeff, his wife Debra and their family also have been unwavering with their love and support.

I get the same pep talk from the Boss, Eddie D. His support has been incredible. So rest assured, I know I'm not alone in this fight.

Every single one of my 49ers teammates that has contacted me has said whatever I need, anytime I need it, they will help. That's just the kind of guys they are. They were so giving as players and now they are the same as friends.

I can't thank my teammates and friends enough for their support. Mr. D always treated us like family and that family is still together. I also want to thank all the great 49ers fans. Your support over the last 35 years has allowed me to remain connected to you. Rarely does a day go by when I'm not asked about 'the Catch,' when we were able to get past the Cowboys and go on to win our first Super Bowl.

I'm not having a press conference or doing any interviews. That time will come. Right now, I've got work to do. I've got to devote all my energy preparing for this battle and I would hope you can respect my family's privacy as I begin this challenge. My ultimate hope is that eventually I can assist in finding a cure for ALS, which disrupts the lives of so many and their loved ones."