In late summer and early fall of 1999, infections were recognized for the first time in the North America. The cause remains uncertain but is likely to be importation of animals, perhaps through the illegal bird trade. Since its original introduction into the New York City area, WNV has caused disease in humans, horses and a wide variety of birds and other vertebrates, spreading into the eastern two thirds of the USA and also into Canada and the Caribbean Basin. The apparent ability of WNV to be disseminated by infected birds and to persist from year to year indicates that it will continue to be a public health problem for the foreseeable future.