Since introduction into the Western Hemisphere, West Nile virus (Flaviviridae: Flavivirus) has established enzootic transmission in nearly every part of the United States and southern Canada and has become an important public health concern. Although approximately 80 percent of individuals infected with West Nile virus are asymptomatic, others have clinical manifestations ranging from febrile illness to neurologic syndromes and possibly death. West Nile virus has a broad host and vector range with competent mosquito and bird vectors, which has led to the establishment of the virus throughout the Western Hemisphere. Although it is primarily transmitted through the bite of an infected mosquito, other modes of West Nile virus transmission have been identified and include infection through transplants of infected organs, transfusion of blood or blood products, intrauterine fetal infection, neonatal infection through breast-feeding, occupational exposure in the field, and laboratory-acquired infections from needlestick or sharps injuries. Controlling the spread of West Nile virus, as well as other emerging vectorborne viral disease, requires laboratory diagnostic testing in conjunction with epidemiologic surveillance, vector control programs, and a variety of personal precautions. No human vaccine is currently available; however, there is an equine vaccine.