An important number of arboviruses are known to produce clinical or subclinical infections in humans. Most of these viruses are maintained in zoonotic cycles and are transmitted by mosquitoes or ticks. Viruses believed to be associated with human disease are classified according to the type of vector, the main clinical sign and the geographic distribution. The arboviruses are classified in families and genera, of which Togaviridae, Flaviviridae and Bunyaviridae are the best known. West Nile virus is present in Egypt, Israel, India and is widespread in parts of Africa, the northern Mediterranean area and Western Asia. The first major West Nile fever epidemic in Europe occurred in Romania, in 1996, with a high rate of neurological infections. 393 patients with serologically confirmed or probable West Nile fever infection (352 had acute central-nervous-system infections) were identified. The number of mild cases could not be estimated. WN virus was recovered from Culex pipiens mosquitoes. The virus is not transmitted through direct human contact, probably the infected mosquitoes transmit the virus throughout their life. Viremia is essential for vector infection and occurs during early clinical illness in humans. Susceptibility appears to be general, in both males and females, throughout life. Inapparent infections and mild disease are common.