Japanese encephalitis: A mosquito-borne flavivirus infection that is the leading cause of viral encephalitis in Asia. Japanese encephalitis virus cannot be transmitted from person-to-person. The Japanese encephalitis virus is related to the viruses of St. Louis encephalitis and Murray Valley encephalitis and to the West Nile virus. Infection leads to overt encephalitis in only 1 of 20 to 1,000 cases, and a vaccine is available. Travelers to Asia from non-endemic countries have a very low risk of infection (less than 1 case per million travelers).