A pandemic of flu (influenza) is when there is a fast spreading, worldwide epidemic. This happens when a completely new strain of influenza develops. It spreads very quickly and affects people badly because it is new. This means that nobody has been exposed to the virus before and as a result nobody has developed natural immunity. It is possible to make a vaccine against the new strain, but this will take a few months from it being identified. In the mean time, the virus spreads like wildfire.

Pandemic flu arises when a strain of bird flu becomes capable of passing from human to human. This is most likely to happen when a person catches avian flu at the same time as human flu. In such circumstances the avian flu might alter its nature (mutate), developing the ability to spread between people. If this happens it is no longer avian or bird flu, but a brand new strain of human influenza, likely to cause a pandemic.

The most famous or notorious pandemic came after the First World War, in 1918-19. This was otherwise known as Spanish Flu and killed up to 50 million people worldwide, more than had been killed, on both sides, in the war.

There have been further, less serious pandemics, in 1957-8 (Asian Flu) and 1968-9 (Hong Kong Flu). All of these have now been shown to have originated from avian flu, and this explains the current concern, around the world, about avian flu.

Most countries have been working on strategic plans for what to do in the event of a pandemic. This has, in the United Kingdom, involved the EU, Central Government, Local Government, local health authorities, poultry farmers, industry and employers in general.

Plans have been put in place to maintain essential services, as well as to provide medical care and advice. Health care will include telephone advice for those who think they may be infected, access to health care services and treatment as necessary. During a pandemic it is likely that routine medical and surgical activities will be curtailed, allowing for the increased workload, and the likelihood that many front line and essential staff would themselves have the virus.

• Medinfo's page on avian flu
• Medinfo's page on influenza
• UK Department of Health Bird flu and pandemic influenza
• Explaining pandemic flu, a guide from the English Chief Medical Officer (October 2005)
• UK Health Protection Agency Influenza Pandemic Contingency Plan
• UK Health Department's Influenza pandemic contingency plan (October 2005)
• US CDC Pandemic influenza: Key facts
• US government information on pandemic flu and avian influenza
• World Health Organisation Ten things you need to know about pandemic influenza
• European Centre for Disease Prevention and Control
• The 1918 Influenza Pandemic