West Nile virus (WNV) is an RNA viral pathogen of the Flaviviridae family that is genetically related to CNS viral pathogens such as dengue, yellow fever, Japanese encephalitis and tick-borne encephalitis. The virus is maintained in the bird population and transmitted through a mosquito vector. When first described in 1940 (Smithburn et al. Am J Trop Med Hyg 1940;1:471-492) in a patient in Uganda, WNV infection produced a mild fever. But the virulence appeared to change in the 1990s with descriptions of severe WNV-associated neurological disease (Hubalek & Halouzka. Emerging Infect Dis 1999; 5:643-650).