A rare chronic progressive encephalitis caused by the measles virus and occurring primarily in children and young adults; death usually occurs within three years; characterized by primary measles infection before the age of two years. Synonyms: Bosin's disease, Dawson's encephalitis, inclusion body encephalitis, sclerosing leukoencephalitis, SSPE, subacute inclusion body encephalitis, subacute sclerosing leukoencephalitis, subacute sclerosing panencephalitis.