Human cells infected with influenza virus, coloured scanning electron micrograph (SEM). Influenza (flu) is a respiratory disease caused by infection with a flu virus. Symptoms can range from mild to severe, commonly including: a high fever, runny nose, sore throat, muscle pains, headache, coughing, and tiredness. Influenza spreads around the world every year causing seasonal outbreaks, resulting in about three to five million cases of severe illness and about 250, 000 to 500, 000 deaths. Swine flu (shown here) was the popular name for a novel virus responsible for a pandemic in 2009-10. Swine flu subsequently transformed into a seasonal virus A/H1N1. It is now one of the seasonal flu viruses that circulate each winter and is included in the flu vaccine. Magnification: x 10000 at 10cm. Specimen courtesy of the influenza research group, Professor Wendy Barclay, Imperial College London, UK.