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The age distribution is given of 79 cases of progressive multifocal leukoencephalopathy confirmed as JC virus infection. The data are compared with a published age distribution of multiple sclerosis onset in Vestfold County, Norway, and with published data from Rochester, Minnesota of the age distribution of viral encephalitis. In contrast to viral encephalitis, 61% of which occurs in children under the age of 10, PML has been identified only once in this age group. PML, like MS, is a disease of adult onset, peaking in the sixth decade of life. The reason for this late onset of a CNS disease caused by a ubiquitous childhood infection is still uncertain, but it may be related to maturation of susceptible cells in the brain, as well as to declining immunity associated with chronic disease and age.

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