H1N1 swine flu virus

A flu primer

Influenza is a potentially serious respiratory disease caused by a family of viruses

Influenza is caused by a virus with an RNA genome. Three types of influenza virus exist – influenza A, B and C. Influenza A is the most common type of influenza virus, and the most diverse.

In the UK influenza affects most people during winter – hence ‘seasonal influenza’. How serious seasonal influenza is depends on which particular strains of virus happen to be circulating.

Periodically, in addition to seasonal flu, major outbreaks (or pandemics) of flu occur, based on novel strains of virus. These can be much more deadly than seasonal flu, as in the 1918 Spanish flu pandemic, although the last pandemic (H1N1 swine flu) in 2009 was relatively mild.

Other species, such as birds, have their own types of influenza virus. These can cause serious disease in humans. But human infections with avian viruses are rare and occur through contact with birds – these viruses are not transmitted between people.

If an avian flu virus did start to be transmitted between people, the consequences could be very serious indeed.

Lead image:

A model of the H1N1 virus, which causes ‘swine flu’. The name H1N1 refers to the types of haemagglutinin and neuraminidase proteins on the surface of the virus. There are roughly ten times as many haemagglutinin proteins (red) as neuraminidase (yellow). In the centre is the single-stranded viral RNA (purple).

Anna Tanczos/Wellcome Images CC BY NC ND

About this resource

This resource was first published in ‘Influenza special issue’ in October 2009 and reviewed and updated in January 2015.

Ecology and environment, Health, infection and disease, Immunology
Influenza special issue
Education levels:
16–19, Continuing professional development