At irregular intervals, an influenza A virus emerges which is different from the current human seasonal influenza viruses and can not only infect humans but can also cause disease in some of them and crucially is capable of efficient human to human transmission. The virus has to be novel enough to prevail over the seasonal A viruses, and because of its novelty there can be little specific immunity among humans, except for older people who may have met a similar virus in the past. This new virus can then spread rapidly from human to human all over the world. Because of the lack of human immunity the virus causes a variable amount of severe disease and deaths: this is an influenza pandemic. As immunity increases among humans, and the pandemic virus changes, the pandemic strain becomes part of (and may dominate) the mix of seasonal influenza A viruses, perhaps changing some of the characteristics of seasonal influenza.
Influenza pandemics vary, and in order to mitigate or even prevent some of their most concerning impacts there is a need for specific and general preparedness. All European Union countries have pandemic preparedness plans and most of them have updated these in the light of the 2009 pandemic experience and of the many evaluations which followed it.
Since 2005 ECDC, the European Commission and the WHO Regional Office for Europe have been providing joint assistance in this work to the European Member States, while the Commission was doing this before 2005 (see the European pandemic preparedness timeline).