Michael J. Fox, who has been suffering from the disease since 1991, tweeted his reaction to the revelation:
"Stunned to learn Robin had PD," he wrote. "Pretty sure his support for our Fdn predated his diagnosis. A true friend; I wish him peace."
Since announcing his diagnosis to the public in 1998, Fox has become a spokesman for people with the disease and has raised millions of dollars through The Michael J. Fox Foundation for Parkinson's Research. Parkinson's is a degenerative nerve disorder that affects nearly 10 million people worldwide.
This wasn't the first time Fox tweeted about the passing of his friend. Upon learning the news Monday, he wrote, "Famously kind, ferociously funny, a genius and a gentle soul. What a loss. #RobinWilliams." (Business Insider)