OCR AS Biology

Revision Notes

4.1.1 Common Pathogens & Communicable Diseases

Common Pathogens & Communicable Diseases

  • A disease is an illness or disorder of the body or mind that leads to poor health
  • Each disease is associated with a set of signs and symptoms
  • Communicable/infectious diseases are caused by pathogens and are transmissible (can be spread between individuals within a population)
  • Both plants and animals can be affected by pathogens

Infectious & Non-infectious Diseases Table

Infectious and non-infectious disease table, downloadable AS & A Level Biology revision notes

  • Many microorganisms are pathogens including:
    • Bacteria
    • Viruses
    • Fungi
    • Protoctists
  • To control disease, it is very important to know what pathogen is causing it


  • Bacteria are a diverse range of prokaryotic organisms
  • Some bacteria are non-pathogenic (they do not cause any disease or damage) while others are pathogenic
  • Pathogenic bacteria do not always infect the hosts of cells, they can remain within body cavities or spaces
  • M. tuberculosis causes tuberculosis (TB) in humans
    • The bacteria infect the lungs, causing a chronic cough and bloody mucus
    • It is a disease often associated with poor hygiene and sanitation
    • M. bovine in cows can also transmit to humans to cause TB
  • N. meningitidis causes bacterial meningitis in humans
    • Very few bacteria can cross the barrier created by the meninges (the tissue that surrounds the brain and spinal cord) however N. meningitidis crosses this barrier to cause acute inflammation
    • Inflammation of the meninges causes symptoms such as fever, headache, neck stiffness and a characteristic rash
  • Ring rot diseases in potato plants are caused by bacterial pathogens
    • The bacteria infect the vascular tissue and prevent the transport of water, causing the plant to wilt and die
    • The infection spreads into the potato tubers where the vascular tissue is arranged in a ring, producing the characteristic black ring of rot


  • Viruses do not have a cellular structure
    • This means they can’t respire, produce ATP, replicate genetic material or synthesise protein
  • They infect host cells and hijack their machinery to replicate their own genetic material and proteins
  • The first virus ever discovered was the Tobacco Mosaic Virus (TMV)
  • TMV infects several plant species
    • It causes a distinct yellowing of the leaves which produces a mosaic pattern
  • Three different influenza viruses infect humans to cause the flu
    • Influenza A, influenza B and influenza C infect the cells that line the airways
    • They cause a high temperature, body aches and fatigue
    • Influenza A is the virus that causes the most cases of flu globally
      • It has a capsid that surrounds 8 single-stranded molecules of RNA
  • The human immunodeficiency virus (HIV) infects specific cells of the immune system
    • It is an enveloped retrovirus
    • The viral enzyme reverse transcriptase produces single-stranded DNA from its viral RNA
    • DNA polymerase synthesises double-stranded DNA from this single-stranded DNA
    • The double-stranded DNA is inserted into the host DNA and can remain inactive for many years
    • Once activated the DNA provirus is used to synthesise new viruses


  • Protists are unicellular eukaryotes
  • Plasmodium falciparum is a protist that causes severe forms of malaria in humans

    • The parasite is spread by mosquitoes
    • Infected individuals experience fever, chills and fatigue
  • P. infestans causes the infamous potato blight
    • The pathogen is unusual as it has some fungal characteristics
    • It is transmitted via spores
    • The first signs of potato blight are small, dark brown marks on the leaves which quickly increase in size and number
    • The protist destroys potato and tomato crops leaving them completely inedible


  • Fungi have a similar structure to plants
    • Their eukaryotic cells have cell walls and large central vacuoles
    • However, instead of being made of separate cells, their bodies consist of filaments known as hyphae
    • These hyphae form a network and spread throughout a host/soil
  • Fungal diseases are much more common in plants than animals
  • Cattle ringworm and athletes foot are fungal diseases that exist on the surface of the skin
  • Fungal diseases in plants tend to be much more serious and can threaten entire crops
  • Black Sigatoka is a fungal disease in bananas
    • It spreads through the leaves of the plant, reducing its ability to photosynthesise
    • The lack of photosynthesis causes parts of the leaf to die; producing black streaks
    • Eventually, the whole leaf dies

Table of Common Pathogens & Communicable Diseases in Humans

Table of Common Pathogens & Communicable Diseases in Humans, downloadable AS & A Level Biology revision notes

Table of Common Pathogens & Communicable Diseases in Plants

Table of Common Pathogens & Communicable Diseases in Plants, downloadable AS & A Level Biology revision notes


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