The Bottom Line: A neurotropic, arthropod-borne flavivirus, West Nile virus is maintained in an enzootic cycle between mosquitoes and birds, however it may also infect horses and humans. In 1999 West Nile virus has spread to North America, Mexico, South America, and the Caribbean. It infects the central nervous system and can cause severe disease in a small minority of infected humans, mostly immunocompromised or the elderly.
Lim, Stephanie, West Nile virus: immunity and pathogenesis. Viruses 2011 vol:3 iss:6 pg:811 -828
Most knowledge in regards to the immune response and pathogenesis has been drawn from mouse studies. How birds and mammals differ in their responses to the disease has not been completely clarified. There are studies suggesting differences in host response between different bird species may influence the outcome of West Nile virus infection.