Influenza A virus particles, coloured transmission electron micrograph (TEM). This influenza virus can infect multiple species and is the most virulent human pathogen of the three influenza types (A, B and C). The virus consists of ribonucleic acid (RNA), surrounded by a nucleocapsid (yellow) and a lipid envelope (brown). In the envelope are two types of protein spike, haemagglutinin (H) and neuraminidase (N), which determine the strain of virus. The natural hosts of the virus are wild birds, which can pass the virus to other species, such as domestic birds, pigs, horses and humans.
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