St. Louis encephalitis (SLE) is a rare mosquito-borne arboviral disease transmitted to humans by the bite of an infected mosquito. In the United States, most SLE disease cases have occurred in eastern and central states. Five cases of SLE have been reported in Wisconsin between 1964 and 2000. Most persons infected with St. Louis virus (SLEV) have no apparent illness. Symptoms of illness include fever, headache, nausea, vomiting, and tiredness. Some people may have neuroinvasive disease that often involves encephalitis, an inflammation of the brain.