West Nile virus particle, illustration. West Nile virus (WNV) causes encephalitis in humans. The WNV belongs to the flavivirus group, which are RNA (ribonucleic acid) viruses that are surrounded by an outer protein envelope. 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.