In the world of healthcare there is a continual reference to microorganisms or microbes. Microbes live in all parts of the biosphere where there is liquid water. Also they are found in all living things, plants and animals. There are more microbes on and inside your body than there are cells that make up your entire body. Whilst many microbes are useful and necessary for everyday life, some are not.  A pathogen or infectious agent or germ is a microorganism that causes disease.

The human body contains many natural defences against some of the common pathogens including physical barriers, the immune system and some "helpful" bacteria present in the human body's normal flora. However, when we are weak and vulnerable particularly in a hospital environment where people are sick or undergoing surgery we need to support our natural defences with additional tools to reduce the risk from pathogens. One of these key tools is the maintenance of sterility of surgical areas and medical instruments. Sterility and its maintenance, together with the prevention of cross-infection, are at the top of any list of critical factors in patient care.

Sterilisation refers to any process that effectively renders any surface, equipment or article free from viable microorganisms.

In practise, it is impossible to prove that all organisms have been destroyed. Therefore Sterility Assurance levels (SAL) are used as a measure of the survival level of microorganisms after terminal sterilisation. In Europe items such as medical devices can only be labelled ‘sterile’ if the chance of an item remaining contaminated after sterilisation is less than or equal to one chance in a million.

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