What is influenza?

Page last updated: 23 February 2015

Influenza (flu) is a potentially life threatening illness. It is a contagious disease of the respiratory tract caused by influenza viruses. Each year, influenza causes serious infection and death around the globe, usually in the winter months (seasonal influenza).

Due to some pre-existing immunity to the seasonal strains of influenza, most people only suffer a self-limiting illness, lasting from a few days to several weeks. Influenza can lead to complications and for some people - the elderly, people with poor immune systems and people with pre-existing respiratory, cardiac and endocrine disease—influenza can be a significant disease and cause death. It can also cause the death of healthy adults and children.

Three different types of influenza viruses infect humans - types A, B and C. Only influenza A and B cause major outbreaks and severe disease. They are included in seasonal influenza vaccines. Influenza C causes a common cold-like illness in children. Only influenza A is known to have been responsible for influenza pandemics.

Influenza A and B viruses have two main proteins on the outside of the virus: the haemagglutinin (H); and the neuraminidase (N) proteins. These proteins are referred to as ‘antigens’ because they are the structures to which our immune system responds. New strains of influenza A and B continually emerge because of the tendency of these H and N antigens to change.

Of the influenza A viruses, only subtypes H1, H2 and H3 have been transmitted easily between humans. Only the H1 and H3 subtypes are currently circulating causing seasonal influenza in humans.

While influenza B is essentially a human disease, influenza A viruses are found in many species. Influenza A occurs as distinct forms or subtypes based principally on their haemagglutinin antigen, of which there are 16 different versions. Water birds are the natural host of influenza A viruses. From time to time influenza A viruses have, however, successfully jumped the species barrier and have become established in other animals, including humans.