An acute or subacute inflammatory process of the central nervous system characterised histologically by multiple foci of perivascular demyelination. Symptom onset usually occurs several days after an acute viral infection or immunization, but it may coincide with the onset of infection or rarely no antecedent event can be identified. Clinical manifestations include confusion, somnolence, fever, nuchal rigidity, and involuntary movements. The illness may progress to coma and eventually be fatal.