West Nile virus (WNV) is caused by infection with a flavivirus. The vector of transmission is the Culex, Aedes, and Mansonia mosquitoes as well as some ticks with birds as intermediate hosts.

The virus is endemic in several temperate climates around the world including North America, Europe, Africa, and Australia. Most cases in these regions occur during the late summer and fall.

Most cases are asymptomatic. Patients usually present with mild illness of fever, myalgias, and an associated macular and papular rash on the upper body. Occasionally, an associated hepatitis or encephalitis can be life threatening. The incubation period is typically 3-14 days. Fever and severe frontal headache, backache, and anorexia may precede the central nervous system (CNS) signs and symptoms of encephalitis (confusion, neck stiffness, cranial nerve palsies, and generalized weakness), for which the mortality rate is 40%. Mild illnesses usually resolve in less than 1 week, but prolonged fatigue is common. Symptoms of encephalitis may persist for weeks or may be permanent.