NEW YORK, NY - APRIL 09: Michael J. Fox and Robin Williams pose backstage at the hit play 'Bengal Tiger at the Baghdad Zoo' on Broadway at The Richard Rogers Theater on April 9, 2011 in New York City. (Photo by Bruce Glikas/FilmMagic)

Days after Robin Williams' death, his wife Susan Schneider revealed the actor had been diagnosed with early stages of Parkinson's Disease. Upon hearing the news, Williams' friend and Parkinson's activist Michael J. Fox took to Twitter to share his reaction.

Fox tweeted to his over 1.1 million followers Thursday evening that he was "stunned" to learn of Williams' condition:

Stunned to learn Robin had PD. Pretty sure his support for our Fdn predated his diagnosis. A true friend; I wish him peace. — Michael J. Fox (@realmikefox) August 14, 2014

According to the website for the Michael J. Fox Foundation for Parkinson's Research, Fox was diagnosed with young-onset Parkinson's in 1991. He went public with his condition in 1998.

Seemingly related to the news but never mentioning Williams by name, the foundation addressed on its blog how depression and Parkinson's are often related:

"Depression is a symptom of Parkinson's disease, separate from the emotional response that comes with a diagnosis," per the FoxFeed Blog, in an entry posted Thursday, August 14. "As many as 50 percent of people with Parkinson's show clinically significant symptoms of depression at some point in their disease course."

As mentioned in Fox's tweet, Williams was a supporter of his foundation. According to its website, the actor and comedian had participated in the annual A Funny Thing Happened on The Way to Cure Parkinson's Gala, which raised nearly $5 million for Parkinson's research each year.

Alongside Williams and Fox, performers at the gala included The Who, James Taylor, Elvis Costello, Sheryl Crow, Tony Bennett, Bon Jovi, John Mayer, Jon Stewart, Stephen Colbert and Wanda Sykes.