Demyelinative diseases of the CNS and peripheral nervous system can be distinguished on the basis of primary mediation by antibody or T lymphocytes (or failure of the T-cell-mediated response) and on the basis of chronicity. The principal mechanisms are autoimmunization to myelin antigens after actual immunization with tissue or infection with cross-reactive viruses or, alternatively, persistent infection of the nervous system (viral or spirochetal) with an associated immune response to the pathogen.