Jackson said over the last three years, he found it increasingly difficult to perform routine tasks, and getting around became more of a challenge.
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"For a while, I resisted interrupting my work to visit a doctor," Jackson said in a news release. "But as my daily physical struggles intensified I could no longer ignore the symptoms, so I acquiesced."
Jackson said after a battery of tests, his physicians identified the issue as Parkinson's.
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"Recognition of the effects of this disease on me has been painful, and I have been slow to grasp the gravity of it," he said. "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 in hopes of slowing the disease's progression."
Jackson also released a Northwestern Medicine letter saying he was diagnosed in 2015 and has since sought outpatient care.
Jackson 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 isn't known and treatments include medications, surgery and physical therapy. The disease itself isn't fatal but people can die from complications.
It was unclear how Jackson's diagnosis will affect his civil rights work through the Rainbow/PUSH Coalition, the Chicago-based civil rights group he founded more than two decades ago.
A protégé of the Rev. Martin Luther King, Jackson was instrumental in guiding the modern civil rights movement, from issues involving voting rights to education. Twice a candidate for the Democratic presidential nomination in the 1980s, he's remained a strong voice in numerous anti-discrimination efforts, including advocating for affordable housing. He's often seen at protests nationwide.
He said Friday 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 Associated Press contributed to this report.