Several influenza pandemics affected the world population over the course of the 20th and early 21st centuries and the threat of the next pandemic looms large. This film explains a number of key aspects of the constant evolutions of influenza viruses and associated impact on public health. Because of the nature of the virus and its threat, the World Health Organization carries out global surveillance of this disease throughout the year, calling on thousands of scientists worldwide. Their exchange of scientific information and virus materials helps to determine which viruses will most probably represent the major threat during the next influenza season, and thus allows twice a year to decide about the composition of the next vaccines. Vaccination is the most effective intervention to reduce the mortality and morbidity of influenza, seasonal influenza epidemics and unpredictable next pandemic. To be prepared for the risk of a pandemic, which can hit anytime, concerted efforts from all public health professionals remain crucial to match the dimensions of such a global public health threat.