Japanese encephalitis is a virus found mainly in India, China, Japan, and South East Asia. It is transmitted by culicine mosquitoes, mainly Culex tritaeniorhynchus. This type of mosquito breeds in rice paddies and other standing water and becomes infected by feeding on domestic pigs and wild birds. Most cases of the diseases are found in rural areas.

The majority of Japanese encephalitis cases occur in children living in Asian countries in rural areas. People most at risk are expatriates in rural areas, active duty military personnel living in affected areas, and residents of rural areas in affected locations. Travelers from the US to Asian countries usually are at very low risk. Globally approximately 35,000-50,000 symptomatic cases are reported per year.

Japanese encephalitis can be asymptomatic or cause mild infection with fever and a headache. It can also cause a severe infection with swelling of the brain, high fever, neck stiffness, disorientation, seizures, stupor, paralysis, coma, and death.

A vaccine is available but is not recommended for people who travel routinely to Asia but only for longer stays in endemic or epidemic areas. See the CDC Travel Vaccines page for more on travel vaccination.