Avian influenza virus particles. Coloured transmission electron micrograph (TEM) of the H7N3 strain of the influenza virus. The virus consists of ribonucleic acid (RNA), surrounded by a nucleocapsid and a lipid envelope, and covered in surface proteins: haemagglutinin (H) and neuraminidase (N). It was first identified in turkeys in Britain in 1963 and, more recently, at a poultry operation in Saskatchewan, Canada, on 27th September 2007. The natural hosts of this virus are usually wild birds, which show few symptoms. It can, however, spread to other species, including humans, where it can cause conjunctivitis and a mild influenza-like illness. Magnification: x96,500 when printed 10 centimetres high.