Avian flu. Coloured transmission electron micrograph (TEM) of influenza A virus particles (red). This is strain H5N1, which usually infects poultry, but began infecting humans in Hong Kong in 1997. Direct bird-human contact is necessary for transmission of the disease. However, the emergence of potent new strains, formed when strains from different animals exchange genetic information, can result in epidemics that kill millions. The blue cells are MDCK (Madin-Darby canine kidney) culture cells, which are used in vaccine production and virus research. Magnification: x29,500 when printed 10cm wide.