What Causes Infectious Disease?
- Junessa Masaya
- Apr 16
- 4 min read
Genetic Technologies in Medicine
What causes infectious disease is a key starting point in NSW Biology Stage 6, Module 7, Infectious Disease. This topic matters because Module 7 specifically asks students to describe infectious diseases caused by pathogens, including microorganisms, macroorganisms and non-cellular pathogens, and to classify pathogens that cause disease in plants and animals.
In this lesson
what a pathogen is
how bacteria, fungi and protozoa can cause disease
how viruses and prions differ from cellular pathogens
the key differences between the main pathogen groups
why classifying pathogens matters in Module 7
What is a pathogen?
A pathogen is a disease-causing agent.
Infectious diseases are caused by pathogens and can be passed from one host to another. The Module 7 syllabus groups pathogens into:
microorganisms
macroorganisms
non-cellular pathogens
For this page, the main focus is on:
bacteria
fungi
protozoa
viruses
prions
Bacteria
Bacteria are cellular pathogens.
What bacteria are
Bacteria are single-celled organisms that can reproduce independently. HSC marking guidance contrasts bacteria with viruses by stating that bacteria are cellular and can live outside a host.
How bacteria cause disease
Bacteria can cause disease by:
invading tissues
reproducing in the host
producing toxins
Example idea
Tetanus is caused by the bacterium Clostridium tetani. HSC material uses tetanus as an example of a bacterial infectious disease.
Fungi
Fungi can also act as pathogens.
What fungal pathogens are
Fungal pathogens are living organisms that can grow on or in a host and cause disease.
How fungi cause disease
Fungi may:
colonise body surfaces or tissues
damage plant tissues
spread by spores
HSC marking guidance notes that fungi can reproduce asexually by producing spores, and these spores can be released and carried to a new host.
Why fungi matter
Fungal pathogens are especially important in agriculture because they can infect plants and reduce crop production. The Module 7 syllabus specifically lists fungal pathogens as examples for investigation.
Protozoa
Protozoa are microscopic pathogens that can infect animals and humans.
What protozoa are
Protozoa are single-celled organisms.
How protozoa cause disease
Protozoan pathogens can:
live inside a host
reproduce within the host
damage cells and tissues
Why protozoa matter
Protozoan diseases are important because they can spread through water, vectors or direct contact, depending on the organism.
Viruses
Viruses are non-cellular pathogens.
A 2024 HSC multiple-choice question specifically identified prions, not bacteria, fungi or protozoa, as non-cellular pathogens, and HSC marking guidance also describes viruses as non-cellular.
What viruses are
Viruses are made of:
genetic material, DNA or RNA
a protein coat
HSC marking guidance describes viruses as a strand of DNA or RNA in a protein coat.
How viruses cause disease
Viruses cause disease by:
entering host cells
using the host cell machinery to reproduce
damaging or destroying infected cells
HSC marking guidance explains that viruses can only reproduce and metabolise in a host cell.
Why viruses are different
Viruses are different from bacteria, fungi and protozoa because they are not cells and cannot reproduce on their own.
Prions
Prions are also non-cellular pathogens.
A 2024 HSC multiple-choice question identified prions as the correct example of a non-cellular pathogen.
What prions are
Prions are infectious proteins.
How prions cause disease
Prions cause disease by affecting the normal shape of proteins in the nervous system, leading to damage.
Why prions matter
Prions are unusual because they:
are not cells
do not contain the structures of living cells
still cause infectious disease
Cellular and non-cellular pathogens
A useful way to compare pathogens is by asking whether they are cellular or non-cellular.
Cellular pathogens
These include:
bacteria
fungi
protozoa
They are made of cells.
Non-cellular pathogens
These include:
viruses
prions
They are not made of cells.
The Module 7 syllabus specifically asks students to classify pathogens and includes both microorganisms and non-cellular pathogens.
Why classifying pathogens matters
Classifying pathogens matters because different pathogen groups:
infect hosts in different ways
reproduce differently
spread differently
are treated and controlled differently
For example:
antibiotics can be used against some bacterial diseases
antivirals are used for viral diseases
The Module 7 syllabus links pathogen classification to understanding disease transmission, prevention and treatment.
Quick comparison table
Pathogen type | Cellular or non-cellular? | Basic feature |
Bacteria | Cellular | Single-celled organisms that can reproduce independently |
Fungi | Cellular | Can grow on or in hosts and may spread by spores |
Protozoa | Cellular | Single-celled organisms that can infect hosts |
Viruses | Non-cellular | DNA or RNA in a protein coat, reproduce only in host cells |
Prions | Non-cellular | Infectious proteins |
Worked example
Exam-style question
Explain one difference between bacteria and viruses as causes of infectious disease.
Worked answer
Bacteria are cellular pathogens that can reproduce independently, while viruses are non-cellular pathogens that can only reproduce inside host cells. This means viruses depend completely on a host cell, but bacteria do not.
Why this works
This answer:
compares two pathogen types directly
uses the key term non-cellular
links structure to reproduction
Common mistakes
Saying all pathogens are cellular.
Mixing up bacteria and viruses.
Forgetting that prions are non-cellular pathogens.
Saying viruses can reproduce independently like bacteria.
Listing pathogen types without explaining how they differ.
Quick quiz
What is a pathogen?
Which of these are non-cellular pathogens: bacteria, fungi, protozoa, viruses, prions?
How are bacteria different from viruses?
What feature helps many fungi spread to new hosts?
Why is it useful to classify pathogens?
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