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West Nile virus (WNV) is in the genus Flavivirus, family Flavioiridae, and is closely related to other members of this genus: Japanese encephalitis virus in Southeast Asia, Murray Valley encephalitis virus in Australia, and St. Louis encephalitis (SLE) virus in North and South America. The principal vertebrate hosts for these arthropod-borne viruses (arboviruses) are wild birds, and the primary vectors are mosquitoes. Little clinical disease or mortality has been reported previously in wild birds from natural infection with these viruses, although significant morbidity and mortality has occurred in humans and domestic animals. West Nile virus (WNV) previously occurred throughout Africa, Middle East, Europe, and the western parts of Asia and was introduced into the United States in New York City (NYC) in 1999. It is still unknown how WNV entered the US, but it quickly became established, causing a human epidemic of 62 cases and an epizootic in the regional bird population, mostly in American crows. The WNV strain introduced was virulent for North American birds and caused significant mortality in crows and related species. This bird mortality was unusual for arboviruses but quickly became a useful sentinel for public health officials to detect the presence of WNV.