Transmission Cycle West Nile Virus is known as an arthropod-born virus, or an arbovirus, and are transmitted and sustained within blood feeding arthropods, such as the mosquito. The disease, while having no apparent effect on the host, is then transmitted as the mosquito takes a blood meal. As the source of infection, the mosquito or other blood feeding arthropod is known as a primary vector. However, because the life cycle of the mosquito is reletively short an additional host is needed to maintain the life cycle of the virus.

The second part of this life cycle involves a non-human, vertebrate host. Frequently these cycles occur undetected until the virus escapes this cycle and infects a species that does not have an immunity to the pathogen. In the case of West Nile, birds often act as the reservoir vertebrate host. Although brids, particularity crows and jays, infected with West Nile can die or become ill, most infected birds do survive. Within these resevoirs the virus will multiply, creating a condition called viremia, where significant concentrations of the virus curculates in the blood stream. This viremia can last for up to 4 days, after which most hosts develop a life-long immunity. It is during this window that additional mosquitos will feed on an infectious host and continue the cycle.

Human Transmission Humans and other animals are infected primarily by mosquitoes although there are cases of transmission through blood transfusion or from mother to infant. Those infected are initially asymptomatic, in other words have no symptoms, the illness appearing in 2 to 14 days, if at all. People, horses and other mammals are not considered ideal reseviors for the virus, due to the fact that while symptoms and disease may occur, there have been no cases of these larger vertabrates developing infectious-level viremias. Because of this these mamalian infections are often labled as "dead-end" or incidental-hosts.

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