West Nile fever is caused by a virus transmitted by mosquitoes and whose reservoir includes wild birds and mosquitoes. Humans are mainly infected through mosquito bites, but infection can occur through organ transplantation and blood.

An incubation period of 3–14 days precedes symptoms. However, most human infections are asymptomatic. The majority of clinical cases are mild and present with flu-like symptoms. Severe cases with signs of encephalitis, meningo-encephalitis or meningitis, are most often observed among elderly. No specific therapy is available.

As it affects countries in Europe every year, West Nile fever is now recognised as a major cause of public health concern in this region. No vaccine is currently available. The main preventive measures are aimed at reducing exposure to mosquito bites.