An illness caused by a virus that is spread through the bites of infected mosquitos.

Japanese encephalitis mainly happens in some parts of Asia and the Western Pacific. Spending lots of time outdoors in rural areas can raise the risk of the illness. The risk also is higher during the summer and fall when mosquitos are more common. A vaccine can help prevent Japanese encephalitis in people at higher risk.

When Japanese encephalitis causes symptoms, the symptoms usually start 5 to 15 days after infection. Early symptoms can include fever, headache and vomiting. Later symptoms can include confusion, seizures and coma. Rarely, brain swelling develops. But most often, Japanese encephalitis causes mild or no symptoms.

Japanese encephalitis has no specific treatment. Rest, fluids and pain medicines sold without a prescription can ease symptoms. Hospital care, breathing support and fluids given through a vein may be needed for severe illness.