The NFL is a very physical sport. Injuries happen all the time. Even after a player's career is over, the effects of playing football often manifest itself even years after retirement. Former San Francisco 49ers' legendary wide receiver Dwight Clark is one of those players.

As ESPN reported, Dwight Clark has been diagnosed with amyotrophic lateral sclerosis or ALS, and according to him, years of playing professional football in the NFL may have caused the disease. Dwight Clark made the announcement last Sunday in a statement posted on the website of Ed Bartolo, Jr., the former owner of the San Francisco 49ers.

ALS, also known as Lou Gehrig's disease, is a neurological disease that mainly involves the nerve cells that's responsible for controlling voluntary muscle movement, according to a report from the National Institute of Neurological Disorders and Stroke. Muscle weakness in the arms, legs, neck or diaphragm is some of the symptoms of ALS. Dwight Clark experienced those symptoms way back in 2015.

According to Dwight Clark, he first started feeling the effect of ALS when he experienced weakness in his left hand back in September of 2015. At first Dwight Clark ignored it as he has been experiencing neck pain constantly during his playing days. Dwight Clark assumed that it was nerve damage because it would just come and go. Test results confirmed what Dwight Clark and everybody in his family feared.

"After months of tests and treatment, I got some bad news," Dwight Clark said. "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."

Dwight Clark also said that football may have an effect on why he contacted the disease. ALS has been linked the degenerative brain disease known as chronic traumatic encephalopathy, or CTE. Studies have shown that athletes that have a history of concussions are mostly likely to have CTE.