This is the family of viruses to which the West Nile Virus belongs. It also contains others such as St. Louis Encephalitis; which was initially thought to be the culprit for the first outbreaks of WNV in the United States.
Flavivirus is a genus of the family Flaviviridae. This complex includes the West Nile virus, dengue virus, Tick-borne Encephalitis Virus, Yellow Fever Virus, and several other viruses which cause encephalitis.