West Nile virus is part of the family Flaviviridae, a large group of RNA viruses. Flaviviridae are distributed worldwide, consisting of about 75 species in 9 serogroups, many members of which cause disease in humans and animals. Important human pathogens include the yellow fever, dengue, West Nile, St. Louis encephalitis, Japanese encephalitis, Murray Valley encephalitis, tick-borne encephalitis, Kyasanur Forest disease, and Omsk hemorrhagic fever viruses.

The WNVs are spherical particles with a membrane envelope and dense core. To replicate, the virus binds to a specific protein on the surface of a cell and enters the cell within a vesicle, a process similar to endocytosis. Replication occurs in the vesicle. The vesicle then fuses with the plasma membrane of the cell and new viral particles are released through the plasma membrane and into the circulatory system.