Japanese encephalitis virus (JEV) is a mosquito–borne agent that can cause encephalitis in equids and humans and reproductive disease in pigs. Rare clinical cases have also been reported in other species, such as cattle. Japanese encephalitis can be a very serious disease in people: although most infections are asymptomatic, clinical cases tend to manifest as severe encephalitis, and many survivors are left with neurological sequelae. All ages can be affected in a population without previous exposure; however, Japanese encephalitis tends to be a childhood disease in endemic areas, where most people develop immunity by the time they reach adulthood. Morbidity and mortality can be high in unvaccinated populations during epidemics. Approximately 4,000 people died during the 1924 epidemic in Japan, and nearly 2500 fatalities occurred in South Korea in 1949. Likewise, more than 3700 equids died during an epidemic in Japan in 1949.

Japanese encephalitis virus has gradually expanded its geographic range within Asia and spread to parts of the western Pacific region during the last 50 years. It could become endemic in additional regions, similarly to West Nile virus, which became established in the Americas in the 1990s. Eradication is unlikely once JEV enters mosquito populations, as the virus is maintained and amplified in cycles between these vectors and various vertebrate hosts such as pigs and wild birds. Vaccination has reduced the number of clinical cases among horses in endemic areas, and is mandatory in certain animals (e.g., racehorses) in some countries. Childhood vaccination has, likewise, greatly decreased the number of human cases in some nations; however, vaccination rates vary, and this disease is still very common in some areas. Some regions have recently reported a relative increase in the percentage of cases seen in adults, leading to suggestions that vaccination campaigns also be conducted in this group.