Key facts Japanese encephalitis is a rare but serious infection caused by a virus spread through mosquito bites It is the leading cause of vaccine-preventable encephalitis in Asia and the western Pacific Most people infected with the virus have no symptoms or mild symptoms A small percentage of people develop inflammation of the brain Overview Japanese encephalitis virus (JEV) is a flavivirus, spread by mosquitoes and related to dengue, yellow fever and West Nile viruses. The virus is transmitted to humans through the bites of infected Culex mosquitoes and is predominantly found in rural parts of Asia and the western Pacific where humans live near pigs, the main animal host. It is thought that around three billion people around the world live in areas where there is a risk of the disease. However, vaccines protect against it. Signs and symptoms Most JEV infections are mild (fever and headache) or without symptoms, but approximately one in 250 infections results in severe clinical illness.…