An opportunistic viral infection of the central nervous system associated with conditions that impair cell-mediated immunity (e.g., acquired immunodeficiency syndrome and other immunologic deficiency syndromes; hematologic neoplasms; immunosuppression; and collagen diseases). The causative organism is JC polyomavirus (jc virus) which primarily affects oligodendrocytes, resulting in multiple areas of demyelination. Clinical manifestations include dementia; ataxia; visual disturbances; and other focal neurologic deficits, generally progressing to a vegetative state within 6 months. (From Joynt, Clinical neurology, 1996, Ch26, pp36-7)