St. Louis encephalitis is caused by a virus with the same name. Mosquitoes are infected with this virus when they feed on birds, and infected mosquitoes can transmit the virus to humans and animals. St. Louis encephalitis is
spread from person to person.
A risk factor is something that increases your chance of getting a disease or condition.
Some factors thought to increase the risk of St. Louis encephalitis include:
- Elderly age
- Living in or visiting the southern, central, or western United States, especially during the
summer and fall