Influenza viruses. Coloured Transmission Electron Micrograph (TEM) of a cluster of influenza (flu) viruses. Each virus is made up of a core of RNA- genetic material (white) surrounded by a spiked protein envelope (green). The tiny external spikes stick to host cells when the virus invades the body. The virus has an affinity for mucus of the respiratory tract, and is spread in droplets from coughs or sneezes. These viruses belong to the orthomyxovirus group. The spikes on the virus envelope can change structure to create a new strain of influenza that may spread as an epidemic. Magnification: unknown.