Human cells infected with influenza virus. Comparison between a scanning election micrograph (SEM left) and transmission electron micrograph (TEM right). Red virus particles bud from the host cell. 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. Magnification: SEM x 13000 TEM x 10000 at 10cm high. SEM specimen courtesy of the influenza research group, Professor Wendy Barclay, Imperial College London. For a series of comparisons between SEMs and TEMs see images C047/7006 to C047/7034.