Parkinson disease is condition of the nervous system that affects movement and gets worse over time. Its symptoms — tremors (often starting in one hand), slowed movement, rigidity — are caused when nerve cells in the brain that make a chemical called dopamine start to break down and die. Dopamine is a neurotransmitter, or messenger, that sends messages to the parts of the brain that control movement. Parkinson disease usually affects people after age 60, but it may start as early as age 40. There is no cure for Parkinson disease, but medications can help reduce the symptoms.