Pandemic flu is caused by a new influenza virus that spreads rapidly causing widespread epidemics in countries around the world. Pandemic influenza occurs when a new, highly infectious and dangerous strain of the influenza virus appears, (in techincal terms, a new influenza A subtype) such as the 'Spanish flu' of 1918-19, in which 40-50 million people worldwide died or the 'Asian flu' of 1957-58 and 'Hong Kong flu' of 1968-69, which each resulted in a million deaths around the world. A pandemic occurs over large geographical areas - usually worldwide - and affects a significant proportion of the population in each country in which it appears.
Pandemics arise when a new virus emerges which is capable of spreading in the world wide population. In the current outbreak of highly pathogenic avian influenza (HPAI) H5N1 the majority of cases have had contact with poultry from which the infection is assumed to have spread. Human infection with HPAI H5N1 has been rare to date and the virus has not acquired the ability to pass easily from human-to-human. This HPAI H5N1 virus could cause a pandemic if it were to combine with a human influenza virus or adapt to the human body and develop the ability to spread efficiently between humans.
No one knows for sure. Influenza viruses are highly unstable and their behavior defies prediction. However, WHO believes that if the right actions are taken quickly, an influenza pandemic might be averted. At present this is WHO's foremost objective.
The first priority, and the major line of defence, is to reduce opportunities for human exposure to the largest reservoir of the virus: infected poultry. This is achieved through the rapid detection of poultry outbreaks and the emergency introduction of control measures, including the destruction of all infected or exposed poultry stock, and the proper disposal of carcasses.
All available evidence points to an increased risk of transmission to humans when outbreaks of HPAI H5N1 are widespread in poultry. As the number of human infections grows, the risk of a new virus subtype emerging increases, triggering an influenza pandemic.
WHO stresses the urgency of the situation and the need for rapid action in the animal and agricultural sectors. For example, the culling in 1997 of Hong Kong 's entire bird population - an estimated 1.5 million chickens and other birds - was done in 3 days. Again in 2003, the culling of nearly 30 million birds (out of a total bird population of 100 million) in the Netherlands was done within a week. Rapid action in both of these situations is thought by many influenza experts to have averted an influenza pandemic in humans.
See the external links on this page for responses to pandemic influenza from the UK government and other partner agencies.