Parkinson's disease is a disorder of certain nerve cells in the brain that normally produce a chemical called dopamine, which helps the brain direct and control movement. In Parkinson's disease, these dopamine-producing nerve cells break down, causing dopamine levels to drop and affecting the brain signals that direct movement.

The classic symptoms of Parkinson's disease are shaking (tremor), stiff muscles (rigidity), and slow movement (bradykinesia). A person with fully developed Parkinson's disease may also have a stooped posture, a blank stare or fixed facial expression, speech problems, and problems with balance or walking, as well as a decline in intellect.

The cause of the disease is unknown. Parkinson's disease usually begins in middle or late life, rarely before age 50—except in cases where genetic causes are suspected. The disease usually progresses gradually over many years, often at different rates in different people. There is no cure for Parkinson's disease, but medication and in some cases surgery can help relieve symptoms.