Parkinson's disease usually occurs in late middle life or beyond. The mean age of onset is 58 to 62. Onset before age 30 is rare but is not unknown, and there is a juvenile form of Parkinson's disease. The greatest incidence is in the decade age 70 to 79 years, with an incidence of 1 to 2 per 1,000 population per year. Later the incidence of parkinsonism seems to decline, a finding which, if true, would have important implications about pathogenesis, indicating that the disease is not simply a result of the operation of the aging process on the nervous system. There appears to be no difference between the sexes in regard to the risk of being affected by Parkinson's disease; early studies in this respect were in error because of decreased ascertainment in females. It is now generally accepted that the ratio is almost or exactly the same for both sexes. Parkinson's disease was known before 1817, the year of publication of the famous manuscript by James Parkinson, but prevalence studies have been possible only since the 1960s, and they indicate no substantial change in incidence.

In 1917 an epidemic of encephalitis lethargica started in Vienna and spread throughout the world. Following this illness, about half of the victims developed Parkinson's disease with tremor, bradykinesia, and rigidity often associated with oculogyric crises, parkinsonian crises (sudden episodic worsening of signs and symptoms), behavioral abnormalities, cranial nerve palsies, and a host of other central nervous system abnormalities. The age of onset of the postencephalitic Parkinson's disease is early compared to other types of Parkinson's disease. A popular and now discarded theory, the Cohort theory, hypothesized that all Parkinson's disease was caused by the encephalitis lethargica agent and that Parkinson's disease would therefore disappear in the 1980s. Since 1961 when the Cohort theory was promulgated, evidence against it has been overwhelming, and we now know that there is no decrease in the prevalence of Parkinson's disease, but of course, the postencephalitic Parkinson's disease has almost disappeared.

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