West Nile virus (WNV) is a virus of the family Flaviviridae part of the Japanese encephalitis (JE) antigenic complex of viruses. Image reconstructions and cryoelectron microscopyreveal a 45-50 nm virion covered with a relatively smooth proteinsurface. This structure is similar to virus; both belong to the genus flavivirus within the family Flaviviridae. WNV is a positive-sense, single strand of RNA, it is between 11,000 and 12,000 nucleotides long which encode seven non-structural proteins and three structural proteins. The RNA strand is held within a nucleocapsid formed from 12 kDaprotein blocks; the capsid is contained within a host-derived membrane altered by two viral glycoproteins.