Influenza B virus, cut-away illustration. Influenza (flu) is a respiratory disease caused by infection with a flu virus. 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 is a type of flu that is similar to influenza A and contributes to the annual seasonal flu outbreak. 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.