West Nile virus (WNV) (Flaviviridae: Flavivirus) is transmitted from mosquitoes 
to birds, but can cause fatal encephalitis in infected humans. Since its 
introduction into North America in New York in 1999, it has spread throughout 
the western hemisphere. Multiple outbreaks have also occurred in Europe over the 
last 20 years. This review highlights recent efforts to understand how host 
pressures impact viral population genetics, genotypic and phenotypic changes 
which have occurred in the WNV genome as it adapts to this novel environment, 
and molecular epidemiology of WNV worldwide. Future research directions are also 
discussed.