Commonly described by the WHO as ‘the worldwide spread of a new disease’, no strict definition is provided. In 2009, they set out the basic requirements for a pandemic:
- New virus emerges in humans
- Minimal or no population immunity
- Causes serious illness; high morbidity/mortality
- Spreads easily from person to person
- Global outbreak of disease.
The US Centre for Disease Control uses a similar approach, but with a reduced set of criteria. It is very difficult to gauge whether the spread of a disease should be termed an outbreak, epidemic or pandemic. In other words, when to declare a pandemic isn’t a black and white decision (Doshi, 2011).