Japanese encephalitis virus (JEV) is the most important cause of epidemic encephalitis worldwide, with an estimated 35,000 to 50,000 cases and 10,000 deaths annually. The virus is a member of the JE serogroup of the genus Flavivirus, family Flaviviridae, and is transmitted between vertebrate hosts by mosquitoes, principally by Culex tritaeniorhynchus. Other important members of the same serogroup of flaviviruses include West Nile virus (WNV), which has recently spread to cause outbreaks of encephalitis in North America, St. Louis encephalitis virus (SLEV), Kunjin virus (KUNV), and Murray Valley encephalitis virus (MVEV). Like other flaviviruses, JEV consists of a small (50 nm) glycoprotein-containing lipid envelope surrounding a nucleocapsid which encloses one molecule of single-stranded positive-sense RNA. This 11-kb molecule comprises 5- and 3-untranslated regions (UTRs), between which lie a single open reading frame carrying genes for three structural proteins (capsid [C], premembrane [PrM], and envelope [E]) and seven nonstructural (NS) proteins