Abstract A very large number of living organisms may invade the nervous system or its membranes, thus causing many forms of encephalomyelitis, meningitis, and meningoencephalomyelitis. The pyogenic organisms mostly cause pyogenic menil1gitis or cerebral abscess. The tubercle bacillus causes tuberculous meningitis or tuberculoma. Treponema pallidum, various yeasts, and moulds also attack the nervous system. Though any of these by invading the brain may cause a disorder which can accurately be described as encephalitis, the process is not usually acute, and the term acute encephalomyelitis is in practice reserved for two groups of disorder, namely those due to the direct invasion of the nervous system by a virus, and those in which, although the primary cause is an infection of the body, usually with a virus, the changes which occur in the nervous system are not directly due to its invasion by the virus, but are the result of some incompletely understood hypersensitivity reaction to the systemic infection, which leads to acute disseminated or demyelinating encephalomyelitis, especially if the patient is immunosuppressed.