Coloured scanning electron micrograph (SEM) of a human cell infected with H3N2 flu virus. Influenza (flu) is an infectious disease caused by an influenza virus. Symptoms can be mild to severe. There are two flu seasons every year, one in each hemisphere, resulting in about three to five million cases of severe illness and about 250,000 to 500,000 deaths. Australian flu (Aussie flu) emerged in 2017 and was particularly severe in the elderly. H3N2 influenza viruses often form long filamentous particles (blue) from the cell. It isn't exactly clear why H3N2 is more virulent than other flu strains but a possible reason is that it mutates faster than other viruses, escaping our immune responses and making it difficult to update the vaccine to match the circulating strain. Magnification: x5000 at 10cm. Specimen courtesy of the influenza research group, Professor Wendy Barclay, Imperial College, London, UK.