Influenza is the most frequent cause of acute respiratory illness requiring medical intervention. It is of global importance and it has caused epidemics and pandemics of human disease for centuries. It affects all age groups and it can recur in any individual. The influenza virus spreads in respiratory droplets from coughing and sneezing.

An example: an airplane with 54 passengers, 1 of whom had influenza, was grounded for three hours and the airplane ventilation system was inoperative during the delay. Influenza developed in 72% of the passengers during the next three days.

The disease usually spreads in certain periods of the year, especially winter or early spring. When healthy people catch influenza, the disease is self-limiting and usually disappears in about one week. In contrast, influenza can cause serious complications and even death in people at risk of complications. Apart from people with underlying disease, also all children less than 2 years old, all pregnant women and all elderly people are at risk of complications.