A progressive disorder due to infection with the JC virus, a human polyoma virus; multifocal areas of demyelination of varying size distributed throughout the CNS but sparing the spinal cord and optic nerves, with characteristic cytologic alterations in astrocytes and oligodendrocytes. Pts often present with visual deficits (45%), typically a homonymous hemianopia, and mental impairment (38%) (dementia, confusion, personality change), weakness, and ataxia. Seizures occur in ∼20% of pts. Almost all pts have an underlying immune disorder or are receiving immunosuppressive therapy. More than 80% of currently diagnosed PML cases occur in pts with AIDS; it has been estimated that up to 5% of AIDS pts will develop PML. Immunosuppressant drugs such as natalizumab have also been associated with PML.