West Nile is a virus most commonly spread to people by mosquito bites. In North America, cases of West Nile virus (WNV) occur during mosquito season, which starts in the summer and continues through fall. WNV cases have been reported in all of the continental United States. There are no vaccines to prevent or medications to treat WNV. Fortunately, most people infected with WNV do not have symptoms. About 1 in 5 people who are infected develop a fever and other symptoms. About 1 out of 150 infected people develop a serious, sometimes fatal, illness. You can reduce your risk of WNV by using insect repellant and wearing protective clothing to prevent mosquito bites.