This is a report of an encephalitis epidemic which occurred on Guam in late 1947 and early 1948. The virus of Japanese B encephalitis was isolated from four of the five fatal cases and serological tests indicated that it was the etiological agent in many others. Concurrent with this epidemic there was an epidemic of mumps with several cases of encephalitis due to this virus. Other patients showed infection with both viruses and in others no identification of the exact virus was accomplished.
These four groups are compared and discussed individually in relation to symptoms, signs, and laboratory findings, and four cases are presented in detail. The pathological findings are presented and emphasis is placed on one patient who clinically and pathologically had a disease indistinguishable from poliomyelitis but in whom the virus of Japanese B encephalitis was isolated. Further stress is laid on the exact diagnosis of the virus present by use of the proper virus studies.