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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).

These include:

  • The



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