H5N1 avian influenza virus particles, coloured transmission electron micrograph (TEM). Each virus particle consists of ribonucleic acid (RNA), surrounded by a nucleocapsid and a lipid envelope (green). The natural hosts of this virus are wild birds, which show few symptoms. However, infected domestic birds suffer a 90-100% mortality rate. Humans that have contact with infected birds can become infected. The first such infection was identified in South-East Asia in 1997, and the virus has steadily spread across the world, with an outbreak in a poultry farm in the UK in 2007. There are fears that the virus may mutate into a human-transmissible form, which could lead to millions of deaths worldwide. Magnification: x670,000 when printed 10cm wide.