Influenza B virus, Coloured 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 range from mild to severe. Influenza spreads around the world every year causing seasonal outbreaks, resulting in about 3 to 5 million cases of severe illness and about 250, 000 to 500, 000 deaths. Influenza B 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. Shown here are the filamentous and round viral particles. Magnification: x20, 000 at 10cm. Specimen courtesy of the influenza research group, Professor Wendy Barclay Imperial College London.