Microorganism

The word pathogen describes what something does: it causes disease in a host. It does not mean that every microorganism is harmful. Many bacteria, fungi and protists are harmless, and some are useful.

Microorganisms, especially bacteria and microscopic fungi, act as decomposers: they feed on dead material and waste and break them into simpler substances.

Unwanted microorganisms must not enter the sample: contamination could produce misleading observations or cultures. Detailed aseptic techniques are considered separately, but their purpose here is to preserve the reliability of the evidence.