Dengue fever is caused by the dengue virus. In the scientific system that classifies viruses, the dengue virus is part of the family "Flaviviridae" and the genus "Flavivirus". Other viruses that belong to the same family and can make humans sick include yellow fever virus, West Nile virus, Zika virus, Japanese encephalitis virus, and tick-borne encephalitis virus. Most of these viruses are spread by mosquitoes or ticks.