1. Subacute sclerosing panencephalitis, progressive rubella panencephalitis, and progressive multifocal leukoencephalopathy

Several slowly progressive diseases of the human CNS are now known to be caused by viruses. Subacute sclerosing panencephalitis (SSPE) is caused by a measles virus that may be sequestered in the cell rather than eradicated; the disease occurs between the ages of 4 and 20. Progressive rubella panencephalitis (PRP) is caused by a rubella virus that persists in the CNS; its onset is about a decade after rubella-virus infection. Progressive multifocal leukoencephalopathy (PML) is caused by a papovavirus that lyses oligodendrocytes; it usually occurs between the ages of 50 and 60. All three disorders are characterized by usually slow mental and motor deterioration; patients with SSPE and PRP also have myoclonic seizures. The pathogenesis of these diseases is not yet understood, and there is no effective treatment. Death occurs within months to years.