This coloured image of influenza (flu) virus particles was taken with a transmission electron microscope. Each particle is roughly spherical and about 200 nanometres in diameter. The outer protein coat or caspid (coloured dark green here) bears a fringe of rigid spines. These allow the virus to attach to its host cell before invading it, and it is this proteinaceous coat that mutates to produce new strains of flu. The core (pale green) is made up of the virus's genetic material, ribonucleic acid (RNA).