Culex is a genus of mosquitoes, several species of which serve as vectors of one or more important diseases of birds, humans and other animals. The diseases they vector include arbovirus infections, West Nilevirus, filariasis, Japanese encephalitis, St. Louis encephalitis, and avian malaria. They occur worldwide except for the extreme northern parts of the temperate zone, and are the most common form of mosquito encountered in some major US cities such as New York.