What is an influenza pandemic?

An influenza pandemic is a planet-wide epidemic caused by a new viral strain that has the following five characteristics:
- It is capable of multiplying in the human body.
- It causes a serious illness or serious complications that can be fatal.
- It is easily transmitted between humans, causing sustained chains of transmission and outbreaks in entire populations.
- It is new and the population has no immunity against it.
- There is no quickly accessible and effective treatment or vaccination against the strain.