Influenza viruses. Coloured transmission electron micrograph (TEM) of a section through influenza (flu) viruses. Flu viruses are orthomyxoviruses, made up of a core of RNA (ribonucleic acid) in a protein coat which is surrounded by an outer spiked envelope (dark blue). The protein spikes stick to host cells in the infected body. This contagious virus has an affinity for mucus and is spread in droplets from coughs or sneezes. The antigen spikes can mutate and change form making the immune system ineffective and resulting in epidemics. To protect those at risk, new influenza vaccines must be prepared and given every year. Magnification unknown.