The virus that caused the pandemic of 1918 is believed to have originated in pigs, while the pandemics of 1957 and 1968 are believed to have originated in birds. Places where birds, pigs and humans live in close proximity are thought to play a particularly important role in creating a favourable environment for antigenic shifts and drifts. It is important to have excellent surveillance in such areas. Its spread is primarily airborne, especially in crowded enclosed spaces. Most people recover completely within 1-2 weeks, however, severe complications can occur, particularly in children, elderly people, and other vulnerable groups.
Bacterial pneumonia is the most common potentially fatal complication.
Viral pneumonia, which is less common but more severe, is another complication of influenza. Vaccination is one of the main influenza prevention methods. However, due to the constantly changing composition of the flu viruses in circulation, the influenza vaccine must be modified each year to match the current viruses. This entails having detailed knowledge about the circulating strains of influenza viruses. In order to do this, an international network for influenza surveillance was created, and WHO became responsible for its administration in 1948.