St. Louis encephalitis (SLE) is a mosquito-borne illness caused by a virus. It is one of a group of such illnesses, including eastern equine encephalitis and LaCrosse encephalitis, which can affect the central nervous system in people and cause severe complications or even death. SLE is found primarily in the Midwest and the southern United States, although occasional cases occur along the East Coast as far north as Connecticut and New York. Travelers to Central and South American and the Caribbean are also at risk of infection.

St. Louis Encephalitis (SLE) Fact Sheet