Year 9 Science Unit 1 · Disease Lesson 2 of 20 45 min SC5-DIS-02

Pathogens and Disease Causes

They are too small to see, yet they shape human history. Pathogens have caused pandemics that killed millions, and they are still among the greatest threats to human health. Understanding your enemy is the first step to defeating it.

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Think First

Before You Begin

Imagine you could shrink to the size of a cell and travel through a drop of pond water or a sneeze droplet.

Write down your answers before reading on:

  • What kinds of tiny living things might you see?
  • How do you think these tiny organisms could make you sick?
  • Why are some germs harmless while others are deadly?
Write your thinking in your book before reading on.

Work mode: Digital — answers typed below

Learning Intentions

Know

  • The five main types of pathogens: bacteria, viruses, fungi, protists, prions
  • How each type of pathogen causes disease
  • The structural differences between bacteria and viruses

Understand

  • Why antibiotics work on bacteria but not viruses
  • How pathogen structure relates to its method of causing disease
  • Why some pathogens are more dangerous than others

Can Do

  • Identify the type of pathogen responsible for a given disease
  • Explain how different pathogens cause harm
  • Compare bacteria and viruses
Key Terms
Bacteria Single-celled prokaryotic organisms. Some cause disease by producing toxins or invading tissues.
Virus Tiny infectious particles that replicate only inside living cells. They hijack the cell's machinery to make copies of themselves.
Fungi Eukaryotic organisms including yeasts and moulds. Some cause infections, especially in moist areas of the body.
Protist Single-celled eukaryotic organisms. Some, like Plasmodium (malaria), are important disease-causing pathogens.
Prion Infectious proteins that cause normal proteins to misfold, leading to brain diseases like Creutzfeldt-Jakob disease.
Toxin A poisonous substance produced by living organisms, especially bacteria.
Core Content
1

Bacteria

The single-celled invaders

Bacteria

Bacteria

Bacteria are single-celled organisms that can live almost anywhere — in soil, water, on your skin, and inside your body. Most bacteria are harmless or even beneficial, but some cause disease.

Disease-causing bacteria harm the body in two main ways:

  • Producing toxins: Bacteria like Clostridium botulinum release poisonous substances that damage cells and tissues.
  • Invading tissues: Bacteria like Streptococcus multiply in the body and damage tissues directly, causing conditions like strep throat.

Bacteria can be treated with antibiotics — drugs that kill bacteria or stop them from multiplying. However, bacteria are becoming increasingly resistant to antibiotics, which is a major global health concern.

2

Viruses

The cellular hijackers

Viruses are not cells — they are tiny infectious particles made of genetic material (DNA or RNA) surrounded by a protein coat. They are much smaller than bacteria and can only replicate inside living cells.

Viruses cause disease by:

  • Invading host cells and injecting their genetic material
  • Hijacking the cell's machinery to make thousands of new virus copies
  • Bursting out of the cell (lysis), killing it and spreading to nearby cells
  • Triggering immune responses that damage tissues

Because viruses use the host cell's own machinery, antibiotics do not work against viruses. Antiviral drugs and vaccines are the main tools for fighting viral infections.

3

Fungi, Protists, and Prions

The other disease-causers

While bacteria and viruses cause the most well-known diseases, three other types of pathogens are also important:

Fungi include yeasts and moulds. Fungal infections typically affect the skin, nails, or mucous membranes. Examples include athlete's foot, ringworm, and thrush. Fungi thrive in warm, moist environments.

Protists are single-celled eukaryotes. The most significant disease-causing protist is Plasmodium, which causes malaria. It has a complex life cycle involving both mosquitoes and humans. Another example is Giardia, which causes diarrhoeal disease through contaminated water.

Prions are infectious proteins that cause normal proteins in the brain to misfold. This leads to devastating neurodegenerative diseases like variant Creutzfeldt-Jakob disease (vCJD), sometimes called "mad cow disease." Prions are extremely unusual because they contain no genetic material at all.

4

Comparing Pathogens

Why treatment depends on the pathogen type

PathogenSizeLiving?How It Causes DiseaseTreatment
Bacteria1-10 micrometresYesToxins, tissue invasionAntibiotics
Virus20-300 nanometresNoHijacks cells, bursts themAntivirals, vaccines
Fungi2-10 micrometresYesTissue invasion, toxinsAntifungals
Protist1-50 micrometresYesTissue invasion, toxinsAntiparasitics
PrionVery smallNoMisfolds brain proteinsNo effective treatment

This table explains why your doctor will not prescribe antibiotics for a cold (viral) but will for a bacterial throat infection. Using the wrong treatment is not just ineffective — it can be dangerous.

Common Misconceptions

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"All bacteria are harmful." No — most bacteria are harmless or beneficial. Your gut contains trillions of bacteria that help digest food and support your immune system. Only a small fraction of bacterial species cause disease.

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"Viruses are just small bacteria." No — viruses are fundamentally different from bacteria. Bacteria are living cells that can reproduce on their own. Viruses are not cells and can only replicate inside host cells.

✗trong>"Antibiotics can treat any infection." No — antibiotics only work against bacteria. They have no effect on viruses, fungi, protists, or prions. Taking antibiotics for a viral infection contributes to antimicrobial resistance.

Australian Context

Australian Pathogen Research

The Doherty Institute: Based in Melbourne, the Peter Doherty Institute for Infection and Immunity is one of Australia's leading centres for pathogen research. Scientists there study how viruses like influenza and SARS-CoV-2 infect human cells, develop new vaccines, and track disease outbreaks across the country.

CSIRO and infectious disease: Australia's national science agency, CSIRO, conducts research on emerging infectious diseases, antimicrobial resistance, and new treatments. During the COVID-19 pandemic, CSIRO scientists were the first outside China to grow the virus in cell culture, enabling critical research worldwide.

Ross River virus: First identified near the Ross River in Townsville, Queensland, in 1959, this mosquito-borne virus remains one of the most common infectious diseases in Australia, with thousands of cases reported annually. Symptoms include joint pain, rash, and fever. There is no specific treatment or vaccine.

✍ Copy Into Your Books

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Bacteria

  • Single-celled organisms
  • Cause disease by toxins or tissue invasion
  • Treated with antibiotics

Viruses

  • Not cells — genetic material + protein coat
  • Hijack host cells to replicate
  • Antibiotics do NOT work

Other Pathogens

  • Fungi: cause skin and lung infections
  • Protists: cause malaria and giardia
  • Prions: misfolded proteins, no cure
Activities
Activity 1

Pathogen Profiles

Research and compare different pathogens.

1 Create a profile for one bacterial disease and one viral disease. Include: name of pathogen, how it causes disease, how it is treated, and one interesting fact.
Answer in your book.
2 Explain why antibiotics cannot treat viral infections like the common cold. Use the concept of cellular structure in your answer.
Answer in your book.
3 A patient has a sore throat. Their doctor refuses to prescribe antibiotics without a test. Explain why this is good medical practice.
Answer in your book.
Activity 2

Size and Scale

Understand the relative sizes of pathogens.

1 Place these pathogens in order from smallest to largest: bacterium, virus, protist, prion, human red blood cell.
Answer in your book.
2 A virus is 100 nanometres wide. A bacterium is 2 micrometres wide. How many times larger is the bacterium than the virus? Show your working.
Answer in your book.
3 Why does the small size of viruses make them difficult to treat compared to bacteria?
Answer in your book.
Multiple Choice
Q

Test Your Understanding

RememberBand 3

1. Which of the following is a virus?

AE. coli
BStreptococcus
CInfluenza
DSalmonella
UnderstandBand 3

2. Antibiotics are effective against:

AViruses only
BBacteria only
CBoth bacteria and viruses
DNeither bacteria nor viruses
RememberBand 3

3. Which pathogen causes malaria?

AA bacterium
BA virus
CA protist
DA fungus
UnderstandBand 4

4. Why can viruses not be treated with antibiotics?

AThey are too small
BThey are not cells and do not have cell walls
CThey reproduce too quickly
DThey are immune to all drugs
RememberBand 3

5. Prions are unusual because they:

AAre the largest pathogens
BContain no genetic material
CCan be killed with antibiotics
DOnly affect plants
Short Answer
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Short Answer Questions

UnderstandBand 4

1. Compare bacteria and viruses in terms of structure, how they cause disease, and how they are treated. Use specific examples. 4 MARKS

Answer in your book.
UnderstandBand 4

2. Explain how protists like Plasmodium and prions cause disease using completely different mechanisms. 4 MARKS

Answer in your book.
EvaluateBand 5

3. Evaluate the statement: "Because most bacteria are harmless, we should not worry about bacterial infections." Use evidence about bacterial diseases and antimicrobial resistance. 4 MARKS

Answer in your book.

Revisit Your Thinking

Go back to your Think First answer. Has your understanding changed?

Update your thinking in your book.

Answers

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MCQ 1

C — Influenza is caused by the influenza virus. E. coli, Streptococcus, and Salmonella are all bacteria.

MCQ 2

B — Antibiotics work by targeting structures found in bacterial cells, such as cell walls. They have no effect on viruses, which are not cells.

MCQ 3

C — Malaria is caused by Plasmodium, a protist transmitted by mosquitoes.

MCQ 4

B — Viruses are not cells and lack the structures (like cell walls) that antibiotics target. Antiviral drugs work by interfering with viral replication instead.

MCQ 5

B — Prions are infectious proteins that cause normal proteins to misfold. Unlike all other pathogens, they contain no DNA or RNA.

Short Answer 1

Model answer: Bacteria are single-celled organisms with cell walls, cytoplasm, and the ability to reproduce independently. They cause disease by producing toxins (e.g., Clostridium botulinum produces botulinum toxin) or invading tissues (e.g., Streptococcus causes strep throat). Bacteria are treated with antibiotics. Viruses are much smaller, not cells, and consist only of genetic material (DNA or RNA) inside a protein coat. They cause disease by invading host cells, hijacking cellular machinery to replicate, and then bursting out to infect more cells (e.g., influenza virus infects respiratory cells). Antibiotics are ineffective against viruses; antiviral drugs or vaccines are used instead.

Short Answer 2

Model answer: Plasmodium (the protist that causes malaria) is a single-celled eukaryotic organism with a complex life cycle involving both mosquitoes and humans. It invades liver cells and red blood cells, multiplying inside them and causing fever, anaemia, and organ damage. Prions, by contrast, contain no genetic material and are not living organisms. They are misfolded proteins that convert normal proteins in the brain into the same misfolded shape. This creates a chain reaction that destroys brain tissue, leading to neurodegenerative diseases like vCJD. While Plasmodium actively invades and replicates in cells, prions passively corrupt the structure of existing proteins.

Short Answer 3

Model answer: This statement is dangerously incorrect. While it is true that most bacteria are harmless or beneficial (e.g., gut bacteria aid digestion), pathogenic bacteria remain a major cause of death worldwide. Tuberculosis (caused by Mycobacterium tuberculosis) kills over a million people annually. Furthermore, antimicrobial resistance is making bacterial infections increasingly difficult to treat. When antibiotics are overused or misused, bacteria evolve resistance, creating "superbugs" like MRSA that cannot be treated with standard antibiotics. Responsible use of antibiotics, hygiene, and vaccination are essential to controlling bacterial diseases, even though most bacterial species are harmless.

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Lesson Game

Pathogen Blaster

Identify and blast different types of pathogens before they infect the host! Learn their features and weaknesses in this action-packed game.

Mark lesson as complete

Tick when you have finished all activities and checked your answers.

Unit 1 overview · Science subject page