West Nile virus particles. Coloured transmission electron micrograph (TEM) of West Nile virus (WNV) particles (blue) next to a host cell (red). This virus causes encephalitis (inflammation of the brain) in humans. The WNV belongs to the group of Flaviviruses, which are RNA viruses that have a core (dark) surrounded by an outer protein coat (blue) with a cubic symmetry. WNV is transmitted by mosquitoes and infects both humans and animals. Symptoms can range from a mild fever to spontaneous bleeding of the skin and circulatory failure, which are often fatal. WNV occurs in Africa, Asia, Europe and North America.