Progressive multifocal leukoencephalopathy.

Progressive multifocal leukoencephalopathy, a demyelinating disease of the central nervous system, is caused by a polyomavirus. This opportunistic virus is demonstrable in affected brain tissue obtained by biopsy or at autopsy. The disease commonly occurs in immunocompromised patients secondary to lymphoproliferative disease, immunosuppressive therapy, autoimmune disorders and acquired immunodeficiency syndrome. The prognosis is poor.