Seasonal influenza, often called ‘the flu’, is an infectious disease of the respiratory tract caused by an influenza virus and spread efficiently from person to person. Outbreaks of seasonal flu follow largely predictable seasonal patterns and occur annually. In the temperate climate zones, they occur predominantly in the winter months and may emerge from late autumn to early spring. A seasonal influenza epidemic in the temperate climate zones usually lasts from six to ten weeks with an average of 2-10% of the population developing clinical influenza. In tropical zones, seasonal patterns may be less pronounced, with more than one peak of infection.