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Influenza B virus, Black and white scanning electron micrograph (SEM) of flu virus being shed from a human cell. Influenza (flu) is a respiratory disease caused by infection with a flu virus. Symptoms can range from mild to severe. 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. Influenza B (shown here) is a type of flu that is similar to influenza A and contributes to the annual seasonal flu burden, making it an important component of the influenza vaccine. Influenza B viruses only pass from human to human unlike type A where there are strains of virus in animals such as bird flu or swine flu. Magnification: x20, 000 at 10cm. Specimen courtesy of the influenza research group, Professor Wendy Barclay Imperial College London. (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE GSCHMEISSNER/SCIENCE PHOTO LIBRARY)