​​St. Louis encephalitis virus (SLEV) is a virus spread by mosquitoes that can make people sick. Most people infected with SLEV don't have symptoms, but in rare cases, SLEV can cause swelling or inflammation of the brain (encephalitis) and lead to death. Older adults are the most likely to get very sick if they are infected. The symptoms and transmission of SLEV are similar to West Nile virus (WNV), but SLEV is less common in California than WNV.

People can get SLEV from the bite of an infected Culex mosquito. A mosquito becomes infected with SLEV after biting a bird that is infected with the virus. SLEV is not spread from person to person.

The best way to help prevent SLEV is to prevent mosquito bites: use mosquito repellent on clothes and exposed skin when going outdoors (especially at dawn and dusk), and wear long sleeves and pants for extra protection. You can also keep mosquitoes from breeding and biting by draining standing water around your home.