Culex in the context of Japanese encephalitis

West Nile virus (WNV) is a single-stranded RNA virus that causes West Nile fever. It is a member of the family Flaviviridae, from the genus Orthoflavivirus, which also contains the Zika virus, dengue virus, and yellow fever virus. The virus is primarily transmitted by mosquitoes, mostly species of Culex. The primary hosts of WNV are birds, so that the virus remains within a "bird–mosquito–bird" transmission cycle. The virus is genetically related to the Japanese encephalitis family of viruses. Humans and horses both exhibit disease symptoms from the virus, and symptoms rarely occur in other animals.

West Nile virus was not named directly after the Nile River, but after the West Nile district of Uganda where the virus was first isolated in 1937.

Western equine encephalitis virus is the causative agent of the relatively uncommon viral disease Western equine encephalitis (WEE). An alphavirus of the family Togaviridae, the WEE virus is an arbovirus (arthropod-borne virus) transmitted by mosquitoes of the genera Culex and Culiseta. WEE is a recombinant virus between two other alphaviruses, an ancestral Sindbis virus-like virus, and an ancestral Eastern equine encephalitis virus-like virus. There have been under 700 confirmed cases in the U.S. since 1964. This virus contains an envelope that is made up of glycoproteins and nucleic acids. The virus is transmitted to people and horses by bites from infected mosquitoes (Culex tarsalis and Aedes taeniorhynchus) and birds during wet, summer months.

According to the CDC, geographic occurrence for this virus is worldwide, and tends to be more prevalent in places in and around swampy areas where human populations tend to be limited. In North America, WEE is seen primarily in U. S. states and Canadian provinces west of the Mississippi River. The disease is also seen in countries of South America. WEE is commonly a subclinical infection; symptomatic infections are uncommon. However, the disease can cause serious sequelae in infants and children. Unlike Eastern equine encephalitis, the overall mortality of WEE is low (approximately 4%) and is associated mostly with infection in the elderly. Approximately 15–20% of horses that acquire the virus will die or be put down. There is no human vaccine for WEE and there are no licensed therapeutic drugs in the U.S. for this infection. The virus affects the brain and spinal cord of the infected host.