Parkinson's disease (also called paralysis agitans) is a degenerative central nervous system disorder. Parkinson's disease is generally a disease of unknown cause, affecting persons over the age of 60. In the United States, about 1% of individuals (1 in 200) over the age of 60 are affected. However, the disease may occur in younger persons, particularly following inflammation of the brain ( encephalitis ) or from poisoning by carbon monoxide, metals, or certain drugs. Men are more likely to be affected than women. Interestingly, incidence of Parkinson's disease is lower in smokers.

Onset of the disease is gradual and progression of symptoms is usually slow. Parkinson's disease may start with mild shaking of the hands or involuntary nodding of the head. Mental abilities usually remain unimpaired. However, as the disease progresses, there can be frequent mood changes, withdrawal, and depression. Dementia eventually appears in about a third of patients. As noted above, Parkinson's disease commonly begins in middle-aged and elderly individuals, affecting a higher percentage of males, but juvenile-onset Parkinson's disease is also known to occur.