Japanese encephalitis (JE) virus is a mosquito-borne flavivirus which was first isolated in Japan in 1935 from the brain tissue of a fatal encephalitis case. The seasonal occurrence of the disease in Japan suggested a vector relationship, and in 1938 the virus was isolated from Culex tritaeniorhynchus mosquitoes. Although minor epidemics of “summer encephalitis” were recorded in Japan as far back as 1870, the disease attracted little attention until the great epidemic of 1924 resulted in 6125 cases and 3797 deaths. In contrast to the endemic pattern of southern Thailand, a predictable pattern of recurrent annual epidemics has been observed since the late 1960s in northern Thailand. Although JE can produce a mild febrile illness, asceptic meningitis, or visceral inflammatory changes, by far the most important pathological manifestation of infection in humans is acute meningomyeloencephalitis. JE virus can rarely, if ever, be isolated from the peripheral blood during the acute illness in humans.