Japanese encephalitis (JE) virus is a mosquito-borne zoonotic flavivirus that infects a wide range of vertebrate species in an enzootic cycle primarily of large waterfowl birds and swine. Horses and humans are considered bystanders to this enzootic cycle and, once infected, dead-end hosts. JE infection in humans can manifest in a spectrum of disease from asymptomatic infection to a mildly febrile symptomatic illness to a life-threatening disease affecting the central nervous system (CNS). The latter is associated with a high morbidity and mortality as well as long-term neurologic sequelae. The pathogenesis and disease severity of JE is discussed elsewhere in this volume. JE is the most common cause of encephalitis in most of Asia and causes an estimated 35,000 cases of encephalitis annually (Igarashi 1992). By all accounts this is an underestimate of the true disease burden of this virus and evidence suggests that this virus is endemic over a far wider region today than 50 years ago, despite the use of an effective vaccine for humans and animals.