Microorganism

  • Reviewed Sep 27, 2026
  • Checked against CDC & WHO

['Pathogen' redirects to here]

A microorganism (or microbe) is a living thing too small to see without a microscope, such as bacteria, viruses, fungi and parasites. Most are harmless and many are helpful. Those that cause disease are called pathogens, or germs. Examples of concern in healthcare:

  • Bacteria: MRSA, Clostridioides difficile (C. diff), E. coli, Klebsiella
  • Viruses: influenza, SARS-CoV-2 (COVID-19), RSV, norovirus, hepatitis B and C, HIV
  • Fungi: Candida auris, Aspergillus

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