Human cells infected with bird flu virus, coloured scanning electron micrograph (SEM). Influenza (flu) is an infectious disease caused by an influenza virus. The virus shown here in red has a gene derived from H5N1, a type of influenza virus that causes a highly infectious, severe disease in birds called highly pathogenic avian influenza (HPAI) or bird flu. Human cases of H5N1 avian influenza occur occasionally, after people become exposed to infected poultry but the virus is not transmitted onwards from person to person. When people do become infected, the mortality rate is about 60%. Influenza viruses constantly undergo genetic changes. It would be a cause for concern should the H5N1 virus become more easily transmissible among humans. Magnification: x3000 at 10cm. Specimen courtesy of the influenza research group, Professor Wendy Barclay Imperial College London