Influenza, commonly called ‘the flu’, is a contagious infection of the respiratory tract, which affects people of all ages. In many cases, flu is short-lived and can resolve on its own. However, in the very young, the elderly, and people with chronic diseases, flu can cause severe illness and life-threatening complications. Flu kills 250,000 to 500,000 people worldwide each year. Find out more about seasonal influenza and what helps protect against influenza.