Avian influenza virus particles. Coloured transmission electron micrograph (TEM) of a H5 strain of influenza virus type A. All five H5 strains (H5N1, H5N2, H5N3, H5N8 and H5N9) cause bird flu. The particles have an enveloped outer coat, called a capsid (purple). The capsid contains haemagglutinin (H) and neuraminidase (N) proteins, which allow the particles to enter the host's cells and reproduce. The first bird flu (H5N1) outbreak was seen in Scottish chickens in 1959. The natural hosts of the virus are wild birds, which show few symptoms, but these can pass the virus to other species, which produce symptoms including conjunctivitis (an eye infection) and a mild flu-like illness. Magnification: x125,000 when printed 10 centimetres wide.