West Nile virus causes sporadic disease in the Eastern hemisphere that is often asymptomatic or mild, whereas in the Western hemisphere, West Nile virus has been associated with illness and profound mortality in many avian species. West Nile virus might have been transported to North America by an infected mosquito or the virus could have entered within a vertebrate host like a bird. Although the most important method of West Nile virus transmission is by Culex species mosquitoes, additional modes of transmission have been identified. West Nile virus has been isolated from almost 300 species of Western birds. The long-term effects on common species such as corvids, sparrows, grackles, finches, hawks, and robins are still being debated. However the potential effect of West Nile virus on small populations or species with limited geographic distribution, such as Hawaiian avifauna, could be much more catastrophic.