Japanese encephalitis is an infection of the brain caused by a virus. The virus is transmitted to humans by mosquitoes. Japanese encephalitis has an incubation period of 5 to 15 days and the vast majority of infections are asymptomatic: only 1 in 250 infections develop into encephalitis. Illness usually begins with sudden onset of fever with gastrointestinal symptoms and headache. Mental status or behavioral changes, focal neurologic deficits, generalized weakness, and movement disorders may develop over the next few days. Seizures are very common among children. Milder forms of disease such as aseptic meningitis or fever with headache can occur, more commonly among adults.