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)

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

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A simple bacterium structure

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

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

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

Viral particle with protein coat, envelope and DNA or RNA

Structure of a typical virus

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

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

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

Non-communicable diseases are non infectious diseases such as

Pathogens (WJEC GCSE Biology: Combined Science)