Pathogens (WJEC GCSE Biology)

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Pathogens

  • A pathogen is any microorganism that causes disease in another organism (e.g. in plants or animals)
  • Many microorganisms are pathogens including:
    • Bacteria
    • Fungi
    • Protists (protoctists)
    • Viruses
  • Not all species within these groups are pathogens, as many bacteria, fungi and protists are harmless and do not cause disease
  • However, all viruses are pathogenic as they can only exist by living inside the living cells of other organisms (or by using these cells to create more viruses)

Bacterial pathogen

  • Pathogenic bacteria do not always infect the hosts of cells, they can remain within body cavities or spaces
  • Toxins produced by the bacteria also damage cells
  • They are small and can reproduce very quickly
  • Bacterial infections include:
    • M. tuberculosis causes tuberculosis (TB) in humans
    • N. meningitidis causes bacterial meningitis in humans
    • Helicobacter Pylori causes stomach ulcers
    • V. Cholerae causes cholera in humans
  • A bacterial cell consists of the following key features:
    • A cell wall
    • Cell membrane
    • Cytoplasm
    • No distinct nucleus

Structure of a bacterium diagram

99Zz19hr_bacterial-pathogen

A simple bacterium structure

Fungal pathogen

  • Fungal diseases are much more common in plants than animals, including humans
  • Fungi can be single-celled or multicellular (with threads of hyphae)
  • The spores they produce allow them to infect other organisms
  • In plants, fungal diseases tend to be much more serious and can threaten entire crops
  • Fungal diseases include:
    • Cattle ringworm and athlete's foot are fungal diseases in animals
    • Black Sigatoka is a fungal disease in bananas
    • Chalara Ash Dieback is a fungal disease that affects ash trees

Protist pathogen

  • Protists are a diverse group of eukaryotic (and usually unicellular) organisms
  • They are parasites which means they need a host to survive
  • Only a small number of protists are pathogenic, but the diseases they cause are often serious
  • Examples of diseases caused by protists are:
    • Plasmodium falciparum is a protist that causes severe forms of malaria in humans
    • P. infestans causes the infamous potato blight

Viral pathogen

  • Viruses are not part of any classification system as they are not considered living things
    • They do not carry out the seven life processes for themselves, instead they take over a host cell’s metabolic pathways so they can make multiple copies of themselves
  • Examples of diseases caused by viruses are:
    • Human immunodeficiency virus (HIV)
    • Coronavirus (COVID-19)
    • Measles
  • Virus structure is as follows:
    • Genetic material/genes (RNA or DNA)
    • A protein coat
    • Some viruses contain an outer envelope as shown in the image below
  • As a result, they can only replicate inside living cells

Structure of a virus diagram

Viral particle with protein coat, envelope and DNA or RNA

Structure of a typical virus

Examiner Tip

You need to know the structure of a bacterium and virus, but you are not required to know the structure of fungi or protists

Communicable diseases

  • Pathogens cause communicable diseases which means they transfer from a diseased host to a healthy organism during infection, in other words, the disease is infectious
  • Pathogens can be passed on from host to host in different ways, including:
    • Direct contact - the pathogen is passed directly from one host to another by transfer of body fluids such as blood or semen (eg HIV, gonorrhoea, hepatitis B & C)
    • Indirect contact - the pathogen leaves the host and is carried in some way to another, uninfected individual

Methods of transmission table

Method of transmission Entry route into the body Examples of diseases spread in this way
Aerosol (droplets in air) Inhalation of tiny droplets carrying pathogens in the air Common cold, influenza
Food or water

Ingesting food or drinking/bathing in water containing pathogens

Cholera, typhoid, dysentery
Contact Walking on or touching contaminated surfaces Athlete's foot, Salmonella (transmitted by flies that land on food)
Insect bites Insects carrying pathogens e.g. mosquitoes Malaria, dengue fever
Body Fluids Fluids passed through sharing needles, breastfeeding or sexual contact HIV, hepatitis

Examiner Tip

Non-communicable diseases are non infectious diseases such as

  • cancer
  • cardiovascular disease
  • malnutrition

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Ruth

Author: Ruth

Expertise: Biology

Ruth graduated from Sheffield University with a degree in Biology and went on to teach Science in London whilst also completing an MA in innovation in Education. She gained 10 years of teaching experience across the 3 key science disciplines and physical education. Ruth decided to set up a tutoring business to support students in her local area. Ruth has worked with several exam boards and loves to use her experience to produce educational materials which make the mark schemes accessible to all students.