• Communicable diseases are caused by one organism that infects another
  • Infective organisms are known as pathogens
  • Pathogens include: bacteria, viruses, protists and fungi.


  • The most common organism for infection
  • Not all bacteria are pathogens, most do not cause disease
  • Bacteria are typically classified using their basic shape and cell wall


  • Viruses are non-living
  • Viruses are a section of RNA surrounded by a protein.
  • Viruses infect cells by inserting its DNA into the host cell
  • The infected cell will then begin to produce viral proteins
  • All natural viruses are pathogens
  • Viruses effect all types of organisms, including bacteria, those are called bacteriophages.


  • Virus attaches to host cell
  • Inserts viral nucleic acid
  • The cell replicates viral nucleic acid
  • The cell synthesises viral proteins
  • assembly of virus particles
  • Lysis of host cell and new bacteriophages released

Protoctista / Protista

  • Eukaryotic cells
  • Exist as single organisms and as colonies
  • A small percentage are pathogenic, they can affect animals and plants (ie, malaria)
  • Often protista require a vector to transfer the disease (eg, mosquitos for malaria)


  • Not usually a large issue for animals, but affects plants massively
  • Eukaryotes, can be unicellular or multicellular
  • Many fungi are saprophytes, meaning they feed off of decaying matter
  • When fungi infect plants, they often target the leaves, preventing photosynthesis
  • Fungi produce many millions of spores that rapidly infect other organisms

Damage to Tissues

  • Viruses
    • Take over cellular metabolism
    • Viral genetic material is inserted into the genetic material of the host cell
    • The host cell then produces more viruses, which burst out, destroying it.
  • Protista
    • They take over a cell, digest the insides and use this as a medium to reproduce, before bursting out.
  • Fungi
    • Digest and destroy living cells.

Producing Toxins

  • Most pathogenic bacteria will produce a toxin, which is a poison.
  • This toxin damages the cells of the host organism, often by breaking down the plasma membrane or inactivating enzymes.
  • Some fungi also produce toxins