Mohammad Ali, formerly known as Cassius Clay, does not have Parkinson's disease. He has Parkinson's syndrome. They are not the same. Ali's syndrome is one that emulates the disease but is not the actual disease. Speculation is that Ali is genetically prone to it and it was exasturbated by a long career in boxing.
Having heard this from many people I can say without a doubt that boxing does not produce this syndrome any more frequently than many contact activities.
In a recent interview Ali was shown in a gym doing his regular workout. When he went to work on the bag you would not have known that he had PS. His abilities are that ingrained.
He is a phenomonal man from a time of great change who stood tall against his entire nation for his personal beliefs and it cost him everything but his soul.