The group of viruses called the California encephalitis group includes La Crosse virus, California encephalitis virus, trivittatus virus, snowshoe hare virus, and Jamestown Canyon virus.

La Crosse virus is likely the most pathogenic member of the group, as most reported cases of infection by the California encephalitis group of viruses are by the La Crosse virus. La Crosse virus is transmitted via the Aedes triseriatus mosquito that is most active during the daytime. The eastern chipmunk, tree squirrel, and fox are mammalian hosts.

Human infections usually occur from late spring to early fall in the central and eastern United States. Patients who live near wooded areas or who participate in outdoor activities that may increase exposure to mosquitoes are at increased risk for this infection.

The incubation period is 5-15 days. Typically, infections are asymptomatic. Symptomatic infections present with headache, fever, and vomiting. Hyponatremia may be seen. Other less common symptoms include seizures, encephalitis, and focal neurological abnormalities. Neuroinvasive disease is uncommon (usually 100 or fewer cases are reported in the United States yearly). Most neuroinvasive cases occur in children.