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

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

Cellular pathogens typically cause disease by acting as parasites – cannabilising the host resources and disturbing homeostasis

• Bacteria: Prokaryotic cells that can reproduce quickly and may compete with host cells for space and nutrition
  

  • Pathogenic bacteria can potentially cause disease by releasing damaging compounds (exotoxins or endotoxins)

• Fungi: Fungal infections typically affect body surfaces and are more common in tropical regions (moist environments)

  • Fungi can be either unicellular (yeasts) or multicellular (moulds) and most pathogenic species are opportunistic (require susceptibility)

• Protozoa: A diverse group of unicellular eukaroytes that sometimes rely on host organisms to complete part of their life cycle
  

• Helminths: Multicellular worms that live within, and feed on, living hosts (they are endoparasites)
  

Acellular pathogens consist of non-living compounds that are capable of replication (resulting in uncontrolled proliferation)

• Viruses: A virus typically consists of an inner core of nucleic acids (DNA or RNA) surrounded by a protein coat (capsid)
  

  • Viruses invade and then commandeer host cells, using the hijacked cellular machinery to reproduce and spread

• Prions: 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 (which then aggregate to form amyloid fibres)

Types of Pathogens