A pathogen is a disease causing agent that disrupts the normal physiology of the infected organism

  • Pathogens can be cellular (e.g. parasites, protozoa, bacteria) or acellular (viruses and prions)

Types of Pathogens



  • Viruses are metabolically inert and incapable of reproducing independently of a host cell (hence are non-living)
  • They typically consist of an inner core of nucleic acid surrounded by a protein coat (capsid)
  • Simpler viruses may lack a capsid (viroids), whilst more complex viruses may possess an external lipid envelope
  • Viruses can be either DNA-based (adenoviruses) or RNA-based (retroviruses)


  • A prion is an infectious protein that has folded abnormally into a structure capable of causing disease
  • Prions can cause normally folded proteins to refold into the abnormal form and hence propagate within a host body
  • Prion proteins aggregate together to form amyloid fibres that cause holes to form in the brain (spongiform encephalopathy)
  • Infectious prion proteins have a higher beta-sheet content, making them more resistant to denaturation and difficult to treat


  • Bacteria are unicellular prokaryotic cells that can reproduce quickly and compete with host cells for space and nutrition
  • Most bacteria are relatively harmless and some may even form mutualistic relationships with hosts (e.g. normal gut flora)
  • Bacteria may cause disease by producing toxic compounds (exotoxins) or releasing the substances when destroyed (endotoxins)
  • As the toxins retain their destructive capacity beyond bacterial death, they are often the cause of food poisoning


  • Disease-causing fungi usually attack the body surfaces, including the skin and mucous membranes
  • They can be categorised according to whether they are unicellular (yeasts) or multicellular (moulds) – most fungi are moulds
  • Moulds consist of branching filaments called hyphae, which may form a mass of invading threads called mycelium
  • Examples of fungal infections include thrush (yeast infection) and athlete’s foot (mould infection)


  • A parasite is an organism that grows and feeds on an organism to the detriment of the host’s survival
  • Parasites can be classed as either ectoparasites (living on the surface of the host) or endoparasites (living within the host)
  • Endoparasites can include microparasites (e.g. single-cell protozoa) or macroparasites (multicellular helmithes)