Epidemiological studies suggest that patients with multiple sclerosis (MS) are exposed to some infectious agent(s) before puberty. The presence of virus-induced demyelination in animal models indicates that demyelination can occur following the trigger of a virus infection. Data regarding the immunological abnormalities to measles virus (MV) and the presence of neurological complications induced by MV infection suggest that MV may be a causative agent of the demyelination observed in MS. Numerous virological studies (e.g., morphological observation, virus isolation, and the search for the MV gene) have been performed, though definite evidence identifying MV as the causative agent has not yet been obtained.