A pandemic outbreak of a disease is an epidemic which occurs worldwide rather than in just one country. In 1918-19 there was a global (pandemic) outbreak of influenza which killed more people than the First World War. Since then there have been two further pandemic outbreaks of influenza which struck millions worldwide. All three pandemics affected about a quarter of the UK's population.
“Most experts believe that it is not a question of whether there will be another severe influenza pandemic, but when.” (The UK Government’s Chief Medical Officer, 2002)
Influenza pandemics are natural phenomenon that have occurred from time to time for centuries – including three times during the last century. They present a real and daunting challenge to the economic and social wellbeing of any country, as well as a serious risk to the health of its population. Pandemic influenza is one of the most severe natural challenges likely to affect the UK. These inter-pandemic years provide a very important opportunity to develop and strengthen our preparations for the potentially devastating impact of an influenza pandemic” (UK Cabinet Office, 2008)
The pandemic of 1918-19 (also called "Spanish 'Flu") is estimated to have killed about 250,000 people in the UK, and as many as 40 million people worldwide. You may think that times have moved on, and today's medical science would prevent such a toll. However, such pandemics always involve a new strain of a disease for which there are no effective drugs available... they must be produced to combat the new virus.
"Preparatory work will be undertaken which should facilitate development of a suitable vaccine when the need arises, but even routine influenza vaccines take several months to manufacture, and there may be additional technical difficulties in the development of a pandemic vaccine because of the particular properties of the virus. This means that it will take time before vaccine can be produced on a large scale and it may not be available at all for the first wave of a pandemic." (UK Influenza Contingency Plan - 2005)