Biology› Year 12› Module 7› Lesson 01

What Is Infectious Disease?

In 2020, a novel coronavirus brought the global economy to a halt within weeks. Understanding why — and how — requires knowing exactly what a pathogen is, what it does, and how it spreads.

25 min 4 dot points 5 MC · 3 Short Answer Lesson 1 of 21
🦠

Choose how you work — type your answers below or write in your book.

Printable worksheet

Download this lesson's worksheet

Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.

Download PDF Open printable version

Think First

Here is a statement many people believe:

"All microorganisms cause disease — they are harmful by nature."

Do you agree or disagree? Write your reasoning. If you agree, explain why. If you disagree, give a specific example that challenges this claim.

Come back to this at the end of the lesson.

Know

  • The definition of infectious disease and pathogen
  • The three categories of pathogen: microorganisms, macroorganisms, non-cellular
  • Examples of each category causing disease in plants and animals
  • How disease transmission data is collected

Understand

  • Why classifying pathogens matters for treatment and prevention
  • How infectious disease differs from non-infectious disease
  • Why most microorganisms are not pathogens

Can Do

  • Classify a given pathogen into the correct category
  • Describe how data on disease transmission is collected
  • Distinguish infectious from non-infectious disease with examples

📚 Know

  • Key facts and definitions for What Is Infectious Disease?
  • Relevant terminology and conventions

🔗 Understand

  • The concepts and principles underlying What Is Infectious Disease?
  • How to explain the reasoning behind key ideas

✅ Can Do

  • Apply concepts from What Is Infectious Disease? to exam-style questions
  • Justify answers using appropriate biological reasoning
Key Terms — scan these before reading
infectious diseasea disease caused by a
WhatInfectious Disease? | HSC Biology Year 12 Module 7 | HSCScience
Herea statement many people believe:
theyharmful by nature
and definitions for WhatInfectious Disease?
and principles underlying WhatInfectious Disease?

Core Content

Misconceptions to Fix

✗

Wrong: Common misconception for this lesson.

✗

Right: Correct understanding with explanation.

Infectious Disease — The Problem of the Invisible Enemy

For most of human history, disease was attributed to bad air, imbalanced humours, or divine punishment. The idea that tiny, invisible living things could enter the body and destroy it was not accepted until the mid-19th century — and even then it met fierce resistance.

An infectious disease is a disease caused by a pathogen — an organism or agent that enters a host and causes harm. This distinguishes it from non-infectious diseases such as cancer, heart disease, or type 1 diabetes, which arise from genetic, environmental, or lifestyle factors rather than an invading organism.

Infectious Disease
Pathogen (bacterium, virus, fungus, etc.)
Yes — can spread between hosts
COVID-19, malaria, tuberculosis, tinea
Targets the pathogen (antibiotics, antivirals)
Non-Infectious Disease
Genetic mutation, lifestyle, environmental factor
No — cannot spread from person to person
Type 2 diabetes, coronary artery disease, melanoma
Targets symptoms or underlying cause (lifestyle, surgery)
Key distinction for the HSC: Infectious diseases are caused by pathogens and can be transmitted. Non-infectious diseases cannot spread between individuals. Questions often test whether students can correctly classify a given condition — always ask: is there an invading organism?
FEATURE Infectious Disease Non-Infectious Disease Cause Pathogen (bacterium, virus, fungus, prion, etc.) Genetic, lifestyle or environmental factor Transmissible? Yes — spreads between hosts No — cannot spread person to person Example COVID-19, malaria, tuberculosis Type 2 diabetes, heart disease, melanoma Treatment Targets pathogen (antibiotics, antivirals, antifungals) Targets symptoms or underlying cause

Infectious vs Non-Infectious Disease

Categories of Pathogens

Pathogens are classified into three broad categories based on their biological nature. This classification matters because each category requires different diagnostic and treatment strategies.

Microorganisms

Microscopic living organisms. Treated as living cells — can reproduce independently (bacteria) or within a host cell (some).

  • Bacteria: Tuberculosis, golden staph, salmonella
  • Fungi: Tinea, candidiasis, aspergillosis
  • Protozoa: Malaria (Plasmodium), giardia, toxoplasma
Macroorganisms

Visible parasites. Large enough to see with the naked eye at some life stages.

  • Helminths (worms): Tapeworms, roundworms, flukes
  • Ectoparasites: Lice, ticks, mites, fleas

Cause disease through physical damage, nutrient competition, and immune activation.

Non-Cellular Pathogens

Not living cells — cannot independently metabolise or reproduce. Require a host.

  • Viruses: COVID-19, influenza, HIV, HPV, measles
  • Prions: Misfolded proteins — BSE (mad cow disease), CJD
  • Viroids: Small RNA molecules — plant pathogens only
Why this matters for treatment: Bacteria are living cells and can be targeted by antibiotics. Viruses are non-cellular — antibiotics have no effect on them. Fungi require antifungal agents. Prions cannot be inactivated by heat or standard sterilisation. Classification directly determines how a disease can be treated.

Pathogens in Plants and Animals

Pathogens infect both plants and animals, though the pathogens themselves and the mechanisms of disease differ between kingdoms. The HSC requires you to be able to classify pathogens causing disease in both groups.

Pathogen TypePlant ExampleAnimal/Human Example
BacteriumCrown gall disease (Agrobacterium tumefaciens)Tuberculosis (Mycobacterium tuberculosis)
VirusTobacco mosaic virus (TMV)COVID-19 (SARS-CoV-2)
FungusWheat stem rust (Puccinia graminis)Tinea (Trichophyton spp.)
ProtozoanPythium root rot (Oomycete — fungus-like protist)Malaria (Plasmodium falciparum)
HelminthRoot-knot nematodes (Meloidogyne spp.)Tapeworm (Taenia solium)
ViroidPotato spindle tuber viroid (PSTVd)Not known to infect animals
PrionNot known to infect plantsBSE (bovine spongiform encephalopathy)
The disease process showing exposure, infection, incubation, symptoms and outcomes

The disease process: from pathogen exposure through infection, incubation, symptoms and final outcome. Understanding each stage is essential for designing interventions.

Collecting Data on Disease Transmission

Understanding how a disease spreads requires systematic data collection. Epidemiologists — scientists who study disease patterns in populations — use several methods to collect primary and secondary data on transmission.

MethodTypeWhat It RevealsExample
Contact tracingPrimaryWho infected whom; transmission chainsCOVID-19 app data tracking spread through workplaces
Case reportingPrimaryIncidence (new cases) over time and locationNotifiable disease registers (e.g. tuberculosis, meningococcal)
Serology surveysPrimaryWho has been exposed (has antibodies) vs who is susceptibleCOVID-19 seroprevalence studies to estimate true infection rate
Historical recordsSecondaryPatterns over time; epidemic curvesDeath records used to reconstruct the 1918 influenza pandemic
Published researchSecondaryMechanism of transmission; risk factorsWHO and CDC disease surveillance reports
Primary vs secondary data: Primary data is collected directly by the investigator (e.g. swabbing patients, interviewing contacts). Secondary data is collected by someone else and then used for analysis (e.g. published case reports, government health databases). The HSC may ask you to identify the data type in a given scenario.
Data Collection Methods Primary Data Secondary Data Contact tracing Case reporting Serology surveys Historical records Published research

Data Collection Methods — Primary vs Secondary

Add screenshot → diagrams/l01-epidemic-curve.png
Real World — COVID-19: A Pathogen That Changed the World SARS-CoV-2 — the virus causing COVID-19 — is a non-cellular pathogen: a single-stranded RNA virus approximately 100 nm in diameter, roughly 1000 times smaller than the width of a human hair. When it emerged in late 2019, contact tracing teams in Wuhan rapidly collected primary transmission data by interviewing patients and mapping cases to common locations, identifying the Huanan Seafood Market as an early cluster. Within weeks, secondary data from published genomic analyses confirmed human-to-human transmission was occurring. By March 2020, the WHO declared a global pandemic — the first since the 2009 H1N1 influenza outbreak. The speed at which SARS-CoV-2 spread globally illustrates why correctly classifying a pathogen and understanding its transmission route is not an academic exercise — it directly determines the response. You will use COVID-19 data in Activity 02 and Short Answer Q3.
Add screenshot → diagrams/l01-sarscov2-structure.png

Common Misconceptions

Misconception: All microorganisms are pathogens.

The vast majority of microorganisms are not pathogens. The human body contains approximately 38 trillion bacterial cells — most are essential for digestion, immunity, and health. Only a small fraction of known bacterial species cause disease. Calling all microorganisms harmful is like calling all chemicals toxic.

Misconception: Viruses are a type of microorganism.

Viruses are non-cellular — they are not living organisms. They cannot metabolise, grow, or reproduce independently. They are genetic material (DNA or RNA) enclosed in a protein coat, requiring a host cell to replicate. Microorganisms are living cells; viruses are not.

Misconception: Infectious diseases are always caused by microorganisms.

Macroorganisms (tapeworms, roundworms, lice) and non-cellular agents (prions, viroids) also cause infectious disease. Prion diseases such as BSE are caused by misfolded proteins — not by any living organism at all.

Infectious vs Non-Infectious
  • Infectious disease: caused by a pathogen, can be transmitted between hosts.
  • Non-infectious disease: caused by genetic, lifestyle, or environmental factors — not transmissible.
  • Pathogen: any organism or agent that invades a host and causes disease.
  • Examples: COVID-19 (infectious); type 2 diabetes (non-infectious).
Pathogen Categories
  • Microorganisms: bacteria, fungi, protozoa — living cells.
  • Macroorganisms: helminths (worms), ectoparasites (lice, ticks) — visible parasites.
  • Non-cellular: viruses (DNA/RNA + protein coat), prions (misfolded proteins), viroids (RNA — plants only).
  • Viruses are NOT living cells — they cannot reproduce independently.
Pathogens in Plants and Animals
  • Both kingdoms affected by bacteria, viruses, fungi, protozoa, helminths.
  • Viroids: plant pathogens only (e.g. potato spindle tuber viroid).
  • Prions: animal pathogens only (e.g. BSE — mad cow disease).
  • Classification matters: determines treatment approach.
Collecting Transmission Data
  • Primary: contact tracing, case reporting, serology surveys (collected directly).
  • Secondary: historical records, published research, WHO/CDC surveillance.
  • Epidemiologists use both types to map transmission patterns and epidemic curves.
  • Data collection drives the public health response to outbreaks.

Activities

ApplyBand 3
Activity 01

Pathogen Classification Diagram

Pattern A — Draw and Annotate

In your book, construct a classification diagram (branching tree) for pathogens. Your diagram must:

  1. Show the three main categories (microorganisms, macroorganisms, non-cellular pathogens) as branches from a central 'Pathogen' node.
  2. Include at least two sub-categories under each main category (e.g. bacteria and fungi under microorganisms).
  3. Label one specific named example of a disease-causing organism at the end of each branch.
  4. For three of your examples, add a short annotation (one sentence) explaining why that pathogen fits in that category — not just naming it.

Type any notes, corrections or additional detail here after completing your diagram.

AnalyseBand 4
Activity 02

Analysing COVID-19 Transmission Data

Pattern A — Structured Data Analysis

The table below shows COVID-19 transmission data collected during the first wave in Australia (March–May 2020).

Proportion of Cases (%)
38
21
19
14
8
Data Collection Method
Contact tracing interviews
Case reporting by hospitals
Case reporting; serology
Case reporting; contact tracing
Contact tracing interviews
  1. Identify which transmission setting was responsible for the greatest proportion of cases. Suggest one reason why this setting was the most common source.
  2. Classify each data collection method listed in the table as either primary or secondary data. Justify your classification for one example.
  3. The 'community (unknown source)' category accounts for 19% of cases. Explain what this suggests about the limitations of contact tracing as a method of collecting transmission data.
  4. Suggest one additional data collection method that could have been used to better understand the 'community' transmission group. Explain what information it would provide.

Write your responses here or in your book.

Interactive: Pathogen Explorer
Interactive: Infectious vs Non-infectious Classifier

Revisit Your Thinking

You were asked whether all microorganisms cause disease. The verdict: this is a significant misconception.

The human body hosts approximately 38 trillion bacterial cells — most are essential symbionts supporting digestion, vitamin production, and immune development. Scientists have identified around 1,400 species of bacteria known to cause human disease out of an estimated one trillion bacterial species on Earth. That is an extraordinarily small fraction. Most microorganisms are decomposers, soil builders, photosynthesisers, or human symbionts.

Furthermore, viruses — which many people consider the archetypal "harmful microorganism" — are not microorganisms at all. They are non-cellular. The term microorganism applies only to living cells.

If you disagreed with the statement, you were correct. If you agreed, the key insight to carry forward is: pathogenicity (the ability to cause disease) is a specific, relatively rare characteristic — not a defining feature of all microscopic life.

Assessment

MC

Multiple Choice

5 random questions from a replayable lesson bank — feedback shown immediately

Short Answer — 10 marks

1. Distinguish between infectious and non-infectious disease. In your answer, refer to the role of pathogens and give one example of each type of disease. (3 marks)

1 mark: definition of infectious disease referencing pathogens and transmission | 1 mark: definition/description of non-infectious disease | 1 mark: one correct example of each

2. Classify the following pathogens into the correct category and provide a reason for each classification: (a) influenza virus, (b) Plasmodium falciparum, (c) the prion causing CJD. (3 marks)

1 mark per correct classification with reason

3. Evaluate the usefulness of contact tracing as a method for collecting data on COVID-19 transmission. In your answer, describe what contact tracing involves, identify one strength and one limitation of this method, and explain how a second data collection method could be used to address the limitation. (4 marks)

1 mark: description of contact tracing | 1 mark: identified strength with reasoning | 1 mark: identified limitation | 1 mark: second method logically addresses the limitation

Answers

SA1: An infectious disease is caused by a pathogen — an organism or agent that invades a host and causes harm — and can be transmitted between hosts, either directly or indirectly. For example, COVID-19 is an infectious disease caused by the SARS-CoV-2 virus, which spreads via respiratory droplets. A non-infectious disease is not caused by a pathogen and cannot be transmitted from person to person; it arises from genetic, lifestyle, or environmental factors. For example, type 2 diabetes is a non-infectious disease caused by lifestyle factors including diet and physical activity, combined with genetic predisposition.

SA2: (a) Influenza virus: non-cellular pathogen. A virus consists only of genetic material (RNA in this case) enclosed in a protein coat. It has no cell membrane, cannot metabolise, and can only replicate inside a host cell — it is not a living cell. (b) Plasmodium falciparum: microorganism (specifically a protozoan). It is a single-celled eukaryotic organism that can carry out all life processes independently and reproduces inside red blood cells. (c) The prion causing CJD: non-cellular pathogen. A prion is a misfolded protein — it contains no nucleic acid and is not a living organism. It causes disease by inducing normal cellular proteins to misfold, but it has no cellular structure whatsoever.

SA3: Contact tracing involves interviewing diagnosed patients to identify people they came into contact with during their infectious period, then notifying and testing those contacts to interrupt transmission chains. A strength is that it provides specific, direct data on transmission routes and settings — for example, confirming whether disease is spreading through households, workplaces, or healthcare settings. A limitation is that contacts can only be identified if the patient recalls them; community transmission from brief or anonymous encounters (e.g. public transport) cannot be traced, leading to unknown-source cases. This limitation could be addressed by serology surveys — testing a population sample for antibodies to estimate how many people have been exposed, even without known contact histories. This provides population-level exposure data even where individual chains cannot be reconstructed.

⚡
Science Jump

Jump Through Infectious Disease!

Climb platforms using your knowledge of what infectious disease is and how it spreads. Pool: lesson 1.

Module overview · Biology subject page · Next checkpoint · Module quiz