The list of celebrities who have largely disappeared from public view because of Parkinson's disease has become familiar to many: boxer Muhammad Ali, former attorney general Janet Reno, actor Michael J. Fox. Pope John Paul II and others have died from the brain disorder. But they are only the most visible of its many victims: today four million people worldwide have the disease, with 500,000 to one million in North America. About 1 percent of the population older than 60 acquires Parkinsons, and as life expectancies climb, the number of victims is predicted to double by 2040. And yet fully half of all patients show symptoms before age 60, in some cases as early as 35 or 40. Medical science is increasingly challenged to find the cause and to develop effective therapies.