West Nile virus is a potentially serious illness that is most often spread through the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds. Infected mosquitoes can then spread the virus to humans and other animals. West Nile virus has been detected in all lower 48 states (but not in Hawaii or Alaska) and outbreaks of disease have been occurring in the United States every summer since 1999.
The first human cases of West Nile virus in Clark County, along with the first positive mosquitoes, were reported in 2004. The health district conducts surveillance of mosquito populations to monitor the level of virus throughout the valley. Every year since then (except 2010), the virus has been found in both humans and mosquitoes. West Nile virus is now endemic in Clark County, which means we expect to see the virus every year during the mosquito season, which occurs April through November.