Transmission electron micrograph (TEM) of West Nile virus (WNV) particles (virions). This virus is known to cause encephalitis (inflammation of the brain) in humans. The WNV belongs to the group of flaviviruses, which are RNA viruses that are surrounded by an outer protein coat (capsid) with a cubic symmetry. WNV is transmitted by mosquitoes and infects both humans and animals. Symptoms can range from a mild fever, most usually seen in world travellers, to spontaneous bleeding of the skin and circulatory failure, which are often fatal.