CHICAGO >> The Rev. Jesse Jackson disclosed publicly today that he has been seeking outpatient care for two years for Parkinson’s disease and plans to “dedicate” himself to physical therapy to slow the progress of the disease.

In a letter to supporters, the 76-year-old civil rights icon said family and friends noticed a change in him about three years ago, and he could no longer ignore symptoms of the chronic neurological disorder that causes movement difficulties.

“Recognition of the effects of this disease on me has been painful, and I have been slow to grasp the gravity of it,” he wrote. “For me, a Parkinson’s diagnosis is not a stop sign but rather a signal that I must make lifestyle changes and dedicate myself to physical therapy.”

Jackson, who declined to be interviewed, also released a letter from Northwestern Medicine confirming his diagnosis and care.

He vowed to use his voice to help find a cure for the disease.

About 60,000 people in the U.S. are diagnosed with Parkinson’s annually, according to the Parkinson’s Foundation.

It can start with tremors, and symptoms generally worsen over time. The exact cause is unknown. Treatments include medications, surgery and physical therapy.

The disease itself is not fatal but people can die from complications. Jackson said Parkinson’s “bested” his father. Noah Lewis Robinson Sr. died in 1997 at age 88 after suffering a heart attack.

It was unclear how his treatment would affect his leadership of the Rainbow/PUSH Coalition, the Chicago-based civil rights group he founded more than two decades ago. 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.

A protégé of the Rev. Martin Luther King, Jackson was instrumental in guiding the modern civil rights movement on a wide variety of issues, including voting rights and education.

Twice a candidate for the Democratic presidential nomination in the 1980s, he has remained a strong voice in numerous anti-discrimination efforts, including advocating for affordable housing. He’s often seen at protests nationwide and has continued regular forums at Rainbow/PUSH’s headquarters.

He said today in the letter that he is also working on a memoir.

“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. “I steadfastly affirm that I would rather wear out than rust out.”

The Rev. Al Sharpton said he spent the last few days with Jackson in New York City.

Jackson “has changed the nation and served in ways in which he never got credit,” Sharpton said in a statement. “We pray for him, just as he fought for us.”