Encephalitis is a primary inflammatory process in the brain, with resulting disturbance of brain function. For clinical purposes, the disease can be said to be infectious or postinfectious. For the most part, the infectious encephalitides are caused by intracellular parasites: viruses and, less commonly, rickettsiae. The great epidemic of encephalitis in the 1920's (the so-called von Economo's type of encephalitis) is believed to belong in the virus infection group, although no agent was ever isolated.1 All of the bacterial, mycotic, treponemal, and parasitic meningitides can cause an associated encephalitis in the underlying brain, but these are generally classified with the meningitides. The postinfectious encephalitides are believed to represent damage to the nervous system as a part of an allergic response to a preceding infection, usually viral. The entity of postvaccinal encephalitis, similar clinically and pathologically to postinfectious encephalitis, is an occasional complication of vaccination with a virus vaccine (i.