Jackson revealed the news in a letter to his supporters, saying he was diagnosed back in 2015 with the Parkison's disease.
"After a battery of tests, my physicians identified the issue as Parkinson's disease, a disease that bested my father".
Parkinson's disease kills vital nerve cells in the brain that create dopamine.
"It's a sad day in my life to know that my friend of all these years has reached that stage", Davis said Friday after a groundbreaking ceremony at the University of IL at Chicago campus. "I tell you it's going to be a great loss if his voice is not able to be heard", he said.
Jackson, a Presidential Medal of Freedom honoree, has been a leader of the civil rights movement in the United States for decades.
Jackson goes on to describe the pain that has come with understanding the realities of the disease, and the passion he feels for slowing the disease's progression, via physical therapy.
Rahm Emanual, the mayor of Chicago, said his "thoughts and prayers" are with Mr Jackson and his family.
Jackson "has changed the nation and served in ways in which he never got credit", Sharpton said in a statement. About 15 percent to 25 percent of Parkinson's patients have a family member with the disease.
Jackson is the father of former U.S. Rep. Jesse Jackson Jr., who was convicted in 2013 in federal court of public corruption charges for misusing campaign funds. "It is an opportunity for me to use my voice to help in finding a cure for a disease that afflicts seven to 10 million worldwide". Jackson has remained active in his advocacy and travels, including traveling to Puerto Rico last month for a hurricane-relief mission and hosting a symposium in Washington, D.C., earlier this week.
"I will continue to try to instill hope in the hopeless, expand our democracy to the disenfranchised and free innocent prisoners around the world", he wrote.
Chicago Tribune reporter John Byrne contributed.