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 usually arises when a strain of bird flu (eg H5N1) or flu from another species (eg pigs, H1N1) becomes capable of passing from human to human. This is most likely to happen when a person catches the animal flu at the same time as human flu. In such circumstances the flu might alter its nature (mutate), developing the ability to spread between people. If this happens it is no longer an animal 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.

In 2009 swine flu was caught by people in Mexico and person to person spread commenced. This is spreading around the world.

Each country has its own plans and systems for dealing with this. The most important things are simple hygiene and common sense, which all of us can manage:

• Cover mouth and nose when coughing or sneezing, using a tissue whenever possible.
• Dispose of dirty tissues promptly and carefully - bag and bin them.
• Maintain good hygiene by washing hands frequently with soap and water.
• Clean hard surfaces (eg kitchen worktops, door handles) frequently using a normal cleaning product.
• Stay at home when ill.

If you become ill, seek advice through your own country's system. Although flu every year causes deaths, at present it seems that the one that is spreading from person to person (a type A Influenza, H1N1) is not causing as severe a disease as some of these new or novel influenza viruses can. Most people will fight it off in the way they fight off other infections, using symptom treatment for fever, headache, aches and pains etc, as for flu.

The H1N1 virus that is circulating is sensitive to Tamiflu (oseltamivir) and Relenza (zanamivir) but is resistant to amantadine.

Except when someone becomes severely ill, usually as a result of complications of the influenza, most people should stay at home while their body deals with the illness. You will not feel like doing much else anyway, and this also reduces the likelihood of spread.

Telephone lines

The CDC information line is +1-800-CDC-INFO.

• Medinfo's page on influenza
• CDC H1N1 flu (swine flu) information
• World Health Organization Pandemic (H1N1) 2009 global alert and response
• WHO Ten things you need to know about pandemic influenza
• European Centre for Disease Prevention and Control
• UK Influenza pandemic contingency plan
• US government information on influenza at flu.gov
• Medinfo's page on avian flu
• The 1918 Influenza Pandemic