The rate of progression

What is it that determines the rate of progression with Parkinson's disease? I often wonder how this strange disease has so many variables, each patient affected in a different way, no two people with the same story. For example, take Michael J. Fox who was diagnosed at age 29, yet here he is today, aged 52 returning to work, starring in a new television show. What has kept Parkinson's at bay all these years - are there extraordinary circumstances we do not know about, a regime of medications along with exercise? How is it possible that he is now able to return to work? The other end of the spectrum, is hearing about those diagnosed and within 7 - 10 years rapidly go down hill and are sadly no longer with us. Were these cases a more severe form of Parkinson's, or is it down to attitude and that some patients simply give up hope?

What puzzles me is the vast difference between patients. Parkinson's is a chronic and progressive disease and yet some people live with mild symptoms for many years, where as others, such as myself, develop difficulties with movement quite quickly. Parkinson's is a particularly peculiar pesky disease - that's for sure!