How are infectious diseases caught?

Lesson 2

How disease is caught and understanding pathogens and their spread?

Learning outcomes & key terms

Innovation and equal access, inspiration for all

  • To understand in depth some pathogens that cause disease
  • Appreciate how disease spreads
  • Understand how disease infects


Any condition that impairs the normal functioning of the body. Occurs when homeostasis can’t be achieved. It is a disruption of normal body functions and homeostasis

Non-infectious/non-communicable disease: a medical condition or disease that is non-infectious. They can be chronic and progress slowly or result in rapid decline and death. includes most cancers, diabetes, hypertension, rheumatoid arthritis, and sickle cell anaemia

Infectious/communicable disease:  an illness resulting from an infection. It is a disease caused by a pathogen (also known as a germ). Examples include HIV, Ebola, chicken pox, the common cold, hepatitis, influenza, Lyme disease, meningitis, strep throat, tetanus, tuberculosis, Zika virus, and many others

To transmit an infectious disease

The invasion of an organism’s body tissues by disease-causing agents, their multiplication, and the reaction of the host tissues to these organisms and the toxins they produce

Common ways to become infected
Through person-to-person physical contact, inhaling a pathogen, eating/drinking contaminated foods, contact with an animal carrying the pathogen, insect bites

Pathogen (germ)
An organism capable of causing disease

Examples of pathogens
Bacteria, viruses, fungi, parasitic worms

Science understanding

Biological sciences

Describing how the requirements for life (for example oxygen, nutrients, water and removal of waste) are provided through the coordinated function of body systems such as the respiratory, circulatory, digestive, nervous and excretory systems.

Explaining how body systems work together to maintain a functioning body using models, flow diagrams or simulations.

Investigating the response of the body to changes as a result of the presence of micro-organisms.

Science as a human endeavour

Scientific understanding, including models and theories, is contestable and is refined over time through a process of review by the scientific community.

Advances in scientific understanding often rely on technological advances and are often linked to scientific discoveries.

People use scientific knowledge to evaluate whether they accept claims, explanations or predictions, and advances in science can affect people’s lives, including generating new career opportunities.

Values and needs of contemporary society can influence the focus of scientific research.

Science enquiry skills

Formulate questions or hypotheses that can be investigated scientifically.

Analyse patterns and trends in data, including describing relationships between variables and identifying inconsistencies.


What causes disease?

Disease is caused by some germs, called pathogens

We are going to look at four main classes of pathogens

  1. Bacteria
  2. Viruses
  3. Fungi
  4. Parasites


Bacteria are living cells that are microscopic in size

They can range from 5 to 20 micrometers in size

A micrometer is a millionth of a metre

Not all bacteria are bad e.g. some bacteria has been used for years (centuries even) for the production of cheese and yoghurt

Pathogenic bacteria include Cholera, Salmonella, Tuberculosis and E.Coli

Once these pathogenic bacteria get inside our body they may release chemicals called toxins. These toxins and our body’s response to them can make us feel sick


Bacteria Cell Structure

Bacteria Cells are simple with four main parts:


  1. Cell Wall: Helps maintain shape and prevents bursting
  2. Bacterial DNA: Controls the division of the cell and how the cell functions
  3. Plasmid DNA: Small pieces of extra DNA, circular in shape that jumps between bacteria
  4. Flagellum: Helps bacteria move, not essential

Types of Bacteria

There are three main types of bacteria cells – scientists use this characteristic to help identify infections

  1. Coccus (Spherical)
  2. Bacillus (Rod)
  3. Spirochete (Spiral)



Viruses are much smaller than bacteria, they are, by definition, not living and can only survive inside another cell

Viruses attack specific cells – the flu virus targets and attacks cells in the respiratory system (throat and lungs)

Once the virus infects a cell, it starts to replicate until it fills the cell and causes the cell to burst

Examples of viruses include: SARS-CoV2, measles, mumps, rubella, influenza, chickenpox and polio

Structure of a Virus

A virus has a simple structure:

  1. RNA or DNA: the grey genetic material is what the virus needs to take over the cell it is infecting – found on the inside of viruses
  2. Protein Coat to attach and penetrate cells: the S (spike) protein is used by SARS CoV2, the cause of COVID-19




Fungi can often be seen with the naked eye. Mould and Athlete’s Foot are examples of fungal infections

One of the most well known forms of fungus is yeast

Yeast is a single celled fungus that is used to ferment beer and cause bread to rise

Yeast can also cause infection

Structure of Yeast

There are different types of yeast but their cells have similar structures:

1. Nucleus: contains genetic material which controls the replication of the cell
2. Cytoplasm: where chemical reactions of the cell take place
3. Cell Wall: keeps the shape of the cell
4. Bud: new yeast cell


Not everything that carries a disease is a micro-organism

A parasite is an organism that lives in or on another organism – the other organism is called the host

Parasites can include head-lice, fleas and tapeworms – they can live inside or outside the body

How do pathogens spread?

Watch the video above with Prof. Robert Booy on, ‘How do pathogens spread”

There are four main ways that pathogens can spread

Direct Contact – by human touching, or breathing in a cough or sneeze, or kissing an infected person or animal

Indirect Contact – by touching an inanimate object like a doorknob or table that has been previously been in contact with an infected person

Insect Bites – for example a mosquito carrying malaria or a tick carrying Lyme disease. When they bite their next person they can pass the pathogen onwards

Food Contamination – bacteria or viruses present in food and water. E.Coli for example spreads through uncooked hamburger or unwashed fruit

How can we help?

Good personal hygiene

You should wash your hands with soap and water for at least 15-20 seconds before you eat and drink. That way you don’t ingest any possible pathogens

15-20 seconds is approximately the time it takes to sing Happy Birthday

Time yourself

Case Study Campylobacter jejuni infection

You’ve just had a bad case of food poisoning

You’ve been infected with Campylobacter jejuni which is a type of bacterium

Symptoms appear within 1-5 days

Campylobacter jejuni is the most common cause of food poisoning and is often found in raw chicken

Bacterial cells can multiply every 20 minutes. So if you start with 2 cells, in 20 minutes you will have 4, after 40 minutes you will have 8, after 60 minutes, 16 cells and so on

So, if you ate the chicken at 7pm and ingested 2 bacteria cells at what time will you have over 250 cells?

The infective dose can be as few as 1 or 2 bacteria

Class activity

Discuss as a class what online sources of information would be the most reliable when researching pathogens

Working in pairs and using reliable sources, research one of the following diseases:

– Giardiasis
– Invasive Pneumococcal Disease
– Norovirus disease
– Athlete’s foot (tinea pedis)


For your chosen disease, answer the following questions, listing the information sources you used, explaining why you felt each was a reliable source

1) What kind of pathogen causes the disease?
2) How does the pathogen enter the body?
3) What are the symptoms?
4) How is it treated?


  • There four pathogens that cause disease; Bacteria, Fungi, Viruses, Parasites
  • Disease can spread through direct or indirect contact, insect bites and food contamination
  • Through good hygiene we can remove a lot of the risk of the spread of pathogens
  • Disease can spread very quickly. Bacteria cells can multiply every 20 minutes


1) The four main classes of pathogen are
a) Bacteria, viruses, fungi, prions
b) Bacteria, viruses, fungi, parasites
c) Bacteria, viruses, fungi, prokaryotes
d) Bacteria, viruses, fungi, eukaryotes


2) Types of bacteria are
a) Coccus (spherical)
b) Bacillus (rod)
c) Spirochete (spiral)
d) All of the above


3) A virus has
a) A nucleus
b) Ribosomes
c) Organelles
d) Genetic material and a protein coat


4) Pathogens can spread by
a) Insect bites
b) Food contamination
c) Coughing and sneezing
d) All of the above