West Nile virus particle, molecular model. This virus consists of an RNA (ribonucleic acid) core of genetic material, surrounded by a nucleocapsid (red, smooth), which is enclosed in an outer protein envelope (multicoloured). Set in the envelope are the E protein (red and yellow trimers), which has a role in the initial attachment of the virus to a host cell, and the M protein (black), which has a role in the formation of the virus particle. West Nile Virus is an arbovirus (arthropod-borne virus) and is transmitted to humans and other mammals from infected birds through the bite of a mosquito vector. Symptoms can range from a mild fever to spontaneous bleeding of the skin and circulatory failure, which are often fatal.