Named for the doctor who identified 'the shaking palsy' in 1817, Parkinson's disease is a progressive neurodegenerative condition in which the body's ability to produce dopamine, a compound that regulates movement, is greatly diminished. It is not fatal, though many people die from complications of the disease: for example, pneumonia resulting from chest muscle rigidity. --Diagnosis: The four hallmark symptoms are tremors, muscle rigidity, slowed movements and poor balance. Usually 60 to 80 percent of dopamine-producing neurons are damaged or dead before the disease is diagnosed. --Treatment: There is no known cure for Parkinson's, but a number of medications alleviate symptoms, particularly in the early stages. A surgical procedure called deep brain stimulation may also offer some relief. --Average patient's age at diagnosis: 55 to 60 --Prevalence: The number of Americans with Parkinson's is unknown, because there is no definitive test for the disease. Most estimates put it between 100 to 200 cases per 100,000 people. A bill introduced by Rep. Chris Van Hollen, D-Md., in March, H.R. 1362, would gather data to determine a more accurate count. --For more information, go to www.parkinson.org or www.pdf.org. 