Flu viruses are divided in to three main groups: influenza A, B and C. Type A viruses are the source of most ‘ordinary’ flu epidemics and have caused all previous pandemics. Whereas influenza B and C viruses infect humans only, influenza A viruses can also infect birds and other animals. This unique ability to jump the species barrier enables influenza A viruses to cause pandemics.

The World Health Organisation (WHO) monitors influenza across the world. Once a new influenza virus has been identified and shown to have pandemic potential, the WHO will announce the various phases of a pandemic and inform national governments.

The UK Government will then put its plans in to action with the Department of Health in the lead.

There is a clear distinction between seasonal flu, avian flu and pandemic flu.

Seasonal flu – Occurs in the UK population as a matter of course – it's ‘ordinary' flu.

Avian flu affects birds – While it has transmitted to people in Asia in very close proximity to birds (eg. poultry workers), the virus does not pass easily, and there is no relation between chickens with avian flu on a farm and a person getting flu in their home.

Pandemic flu affecting humans – Occurs due to the emergence of a new flu virus which is markedly different from recently circulating strains.