H5N1 avian influenza virus particles, coloured transmission electron micrograph (TEM). Each virus particle (orange) consists of a core of ribonucleic acid (RNA), surrounded by a nucleocapsid and a lipid envelope. The natural hosts of this virus are wild birds, which show few symptoms. However, infected domestic birds suffer a 90100% mortality rate. Humans that have contact with infected birds can become infected. The first such infection was identified in SouthEast 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.