Either clinicians have become more observant of various types of infections of the central nervous system in the last decade, or there has been a decided increase in the number of such infections. Since the pandemic of encephalitis (von Economo's disease), postvaccinal encephalitis has become a definite entity; encephalitis following measles, variola and varicella has been called to attention, and disseminated encephalomyelitis has definitely increased (Redlich1).
In addition to controlling the clinical diagnosis, the neuropathologist attempts to discover the pathogenesis of the disease that he is studying. His rôle in establishing the infectious nature of a syndrome should be invaluable in the search for the etiologic agent. That he cannot fulfil this function is apparent to the critical-minded, since there exists no degree of unanimity of opinion as to whether any of these forms results from toxic, degenerative or infective virus causes. Yet these encephalitides have been compared with