A microorganism is a single-celled organism that's too small to be seen with a microscope.
They include bacteria, viruses and fungi.
A disease-causing microorganism is called a pathogen.
A disease is a malfunction of the body/mind which has a negative effect on health.
- Gain entry to the host
- Colonise host tissues
- Raises defenses of the host
- Causes damage to the host tissues.
Pathogens gain entry to the host via interfaces (surface linking two systems, in this case the external and internal environment).