An example of this vector-host relationship can be observed in the transmission of the West Nile virus. Female mosquitoes of the genus "Culex" prefer to consume the blood of passerine birds, making them the hosts of the virus. When these birds are infected, the virus amplifies, potentially infecting multiple mosquitoes that feed on its blood. These infected mosquitoes may go on to further transmit the virus to more birds. If the mosquito is unable to find its preferred food source, it will choose another. Human blood is sometimes consumed, but since the West Nile virus does not replicate that well in mammals, humans are considered a dead-end host.