[West Nile virus infections: overview and epidemiological update]

West Nile virus, a flavivirus transmitted by mosquitoes, has been intensively studied since a few years because of epidemics/epizootics it has caused the last ten years, in particular around the Mediterranean basin and on the North-American continent. This virus mainly circulates in birds ; migrating bird species disseminate the virus while resident species could play a role in viral cycle amplification. A large number of mammal, amphibian and reptile species can also be infected. This virus can cause a lethal disease in humans and horses. For this reason, an active and/or passive surveillance is carried out in France and in United States at different steps of the transmission cycle : insects, birds, horses and humans. This surveillance is aimed at precociously detecting viral circulation and, if detected, take suitable information, prevention and fight measures. Furthermore, the description of new transmission routes of infection has led to precaution measures for blood and organ donations in the US and in a lesser extent, in France. As West Nile epidemiology is only partially known, most of epidemics remain unpredictable and difficult to control.