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
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
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