Influenza virus. Artwork depicting the anatomy of an influenza virus (group: Orthomyxovirus), such as the Beijing flu virus. At centre, the virus contains a core of RNA (ribonucleic acid) genetic material (green) surrounded by a layered coat or capsid. Purple spheres in the coat consist of protein units; external to these is a lipid envelope (yellow-orange). Influenza viruses have a fringe of surface spikes: haemagglutinin (blue), which adhere to the host cell, and neuraminidase (red). It is these antigen spikes that can mutate and change form to neutralise the host's immune system, so causing different influenza epidemics such as the 1993 outbreak of Beijing flu.

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