The encephalitis problem has many angles. It is easy enough to call attention to the points involved but practically impossible to present any satisfactory answers.
For nearly twenty years there have been outbreaks in various parts of the world of acute infections of the central nervous system, exclusive of poliomyelitis and meningitis. These have differed in seasonal incidence, in age distribution, in case fatality and in clinical manifestations. They have usually been described as some form of acute encephalitis or encephalomyelitis. In conjunction with these more or less spontaneous cases there has been a marked increase in the reports of encephalitis following vaccination, measles and other acute infections. While it is now known that encephalitis may run a mild course in the acute stage and escape recognition, the symptoms in the more severe cases are so arresting that they could hardly have passed unnoticed. It is not probable that clinical