Influenza A virus, strain H5N1. Coloured transmission electron micrograph (TEM) of influenza A, strain H5N1 virus particles (dark brown) bursting from a host cell (light brown). This strain usually infects poultry; it killed millions of chickens in southern China in 1997. It also infected humans for the first time in this year, killing around 6 people in Hong Kong. Influenza is transmitted in water, by direct contact, or by coughing and sneezing. The emergence of potent new strains, formed when strains from different species exchange genetic information, can result in epidemics which kill millions. Magnification: x40,000 at 6x6cm size.