These maps show data on the number of cases of human illness from WNV, the rate of illness per population in the county (incidence), and the mosquito infection rate for each year for which data were reported to the Illinois Department of Public Health. For the incidence rate, the number of human cases have been normalized by population to provide a more sensitive indicator of the human threat. The interpretation of the incidence rate is important, because the number of human cases is naturally related to the number of people who live in the county. For instance, in 2007 and 2011 the human incidence rate was higher in southern Illinois than in northeastern Illinois, though the number of cases was higher in the north. These maps are intended to provide information on the number of cases that might be expected in any given region for high and low incidence years. Note however, that not all counties test for West Nile virus in mosquitoes, especially in regions outside of the more heavily populated counties of northeastern Illinois, so it is important to keep this in mind when reviewing these maps.