How are infectious diseases caught?

Lesson 2

How disease is caught and understanding pathogens, spread and infections?

Learning outcomes & key terms

Innovation and equal access, inspiration for all.

  • To understand in depth four pathogens that cause disease
  • Understand how disease spread
  • 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 thousandth of a meter.

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

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

Once these pathogenic bacteria get inside our body they release chemicals called toxins. These toxins and our body’s response to that 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, jumps between bacteria.
  4. Flagellum: Helps bacteria move, not essential for all bacteria

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 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 cells and causes the cell to burst.

Examples of viruses include: measles, mumps, rubella, influenza, chickenpox and polio.

Structure of a Virus

A virus has a simple structure:

  1. DNA: the genetic material is what the virus needs to take over the cell it is infecting
  2. Protein Coat: to attach and penetrate cells




Fungi can be seen with the naked eye. Mould and Athletes Feet 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 headlice, 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 touching 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 pass the pathogen on.

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

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. That way you don’t eat 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 bacteria. You only need 400-5—bacteria cells to get sick.

Symptoms appear within 2-5 days.

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

Bacteria 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 80 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 400 cells?

We call this the infective dose.


  • There four pathogens that cause disease; Bacteria, Fungi, Virus, 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 celss can multiply every 20 minutes.