Influenzavirus A. Coloured transmission electron micrograph (TEM) of the influenza A virus, which can infect multiple species and is the most virulent human pathogen of the three influenza types. The virus consists of ribonucleic acid (RNA), surrounded by a nucleocapsid and a lipid envelope (dark pink). In the envelope are two types of protein spike, haemagglutinin (H) and neuraminidase (N), which determine the strain of virus. The natural hosts of this virus are usually wild aquatic birds, which show few symptoms. However, from these wild birds, other species (such as domestic birds, pigs, horses and humans) can become infected. Magnification: x43,200 when printed 10 centimetres wide.