Parkinson's disease (PD) is a chronic and progressive movement disorder. The cause is unknown, but it involves the malfunction and death of vital nerve cells in the brain, called neurons. PD primarily affects neurons in an area of the brain called the substantia nigra. Some of these dying neurons produce dopamine, a chemical that sends messages to the part of the brain that controls movement and coordination. As PD progresses, the amount of dopamine produced in the brain decreases, leaving a person unable to control movement normally. Main symptoms of PD include slowness, tremor and gait or balance problems. PD appears mostly in elderly over 60 years old, but approximately 10% of sufferers are below the age of 40.