Birds: The Reservoir (Host)

Infected mosquitoes transmit the virus to birds. According the Centers for Disease Control and Prevention (CDC), at least 138 species of dead birds have tested positive for West Nile Virus. Birds of some species get ill and die, while others become infected but do not show signs of the disease. Once infected, however, birds can pass on the disease to mosquitoes for up to 5 days.
Bird standing on wood post