The term pandemic signifies the extraordinary spread of a human disease on an global scale which rapidly affects all parts of the world and over 25% of the global population.
The twentieth century saw three significant pandemics:
- the pandemic of 1918/1919, known as the 'Spanish flu', which caused over 40 million deaths in under one year;
- the pandemic of 1957, known as the 'Asian flu', which killed over two million people;
- the pandemic of 1968, or the 'Hong Kong flu', which caused one million deaths and spread more slowly than previous pandemics.
A pandemic occurs when a new virus appears against which the human immune system has no defence.