“…In most cases the associated virus is either a nonconventional agent (4)(5)(6)(7) or a common virus with altered properties (10,11,14,19,20). Isolation of virus from infected central nervous system tissue required sophisticated virological procedures in those cases in which it was accomplished (11,12,15,20,21). In virtually all of the diseases, onset of neurological signs is delayed months or years after initial viral infection and the clinical course is prolonged (4-8, 13, 14, 16, 17).…”