Misfolded proteins. No nucleic acid, no cell structure.

A prion induces normal cellular proteins (often PrP in nervous tissue) to misfold into the same abnormal shape, creating aggregates that destroy brain tissue.

Influenza A (RNA virus, respiratory), HIV (retrovirus, immune cells), tobacco mosaic virus (plant pathogen affecting tomato and tobacco leaves).

What are generic disease names?

"A virus" or "a bacterium" scores no marks. Use full scientific names where possible.

§-Syllabus dot point

NSWBiologySyllabus dot point

Inquiry Question 1: How are diseases transmitted?

Describe a variety of infectious diseases caused by pathogens, including microorganisms, macroorganisms and non-cellular pathogens, and collect primary and secondary-sourced data and information relating to disease transmission, including: classifying different pathogens that cause disease in plants and animals

A focused answer to the HSC Biology Module 7 dot point on the causes of infectious disease. Covers prions, viruses, bacteria, protozoa, fungi and macroparasites, with a named example for each and the structural features markers expect.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-21

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The answer
Examples in context

What this dot point is asking

NESA wants you to classify the six main types of pathogen, describe a defining structural feature of each, and give at least one named example of a disease they cause in plants or animals. This is foundational content that appears in multiple choice every year and underpins almost every Module 7 extended response.

The answer

A pathogen is any biological agent that causes disease in a host. Pathogens fall into six categories: prions, viruses, bacteria, protozoa, fungi and macroparasites. The first two are non-cellular; the last four are cellular. The diagram below shows each type with its defining feature and its place on a size scale spanning roughly six orders of magnitude.

Prions

Structure: Misfolded proteins. No nucleic acid, no cell structure.
Mechanism: A prion induces normal cellular proteins (often PrP in nervous tissue) to misfold into the same abnormal shape, creating aggregates that destroy brain tissue.
Example: Bovine spongiform encephalopathy (BSE, "mad cow disease") and the human variant Creutzfeldt-Jakob disease.

Viruses

Structure: Acellular particles. Nucleic acid (DNA or RNA) enclosed in a protein capsid, sometimes with a lipid envelope. Not considered living.
Mechanism: Cannot replicate independently. Inject genetic material into a host cell and hijack the host's machinery to produce new viral particles.
Examples: Influenza A (RNA virus, respiratory), HIV (retrovirus, immune cells), tobacco mosaic virus (plant pathogen affecting tomato and tobacco leaves).

Bacteria

Structure: Prokaryotic single-celled organisms. Cell wall (peptidoglycan), plasma membrane, cytoplasm, 70S ribosomes, circular DNA, often with plasmids. No nucleus.
Mechanism: Cause disease by producing toxins (e.g. Clostridium tetani releases tetanospasmin) or by colonising and damaging host tissue.
Examples: Mycobacterium tuberculosis (tuberculosis), Vibrio cholerae (cholera), Agrobacterium tumefaciens (crown gall in plants).

Protozoa

Structure: Single-celled eukaryotes. Have a nucleus, membrane-bound organelles and often complex life cycles.
Mechanism: Often transmitted by vectors. Invade specific tissues and reproduce inside host cells.
Examples: Plasmodium falciparum (malaria, transmitted by Anopheles mosquito), Trypanosoma brucei (African sleeping sickness, tsetse fly).

Fungi

Structure: Eukaryotic, either unicellular (yeasts) or multicellular (moulds with hyphae). Cell walls made of chitin.
Mechanism: Often opportunistic, infecting compromised tissue or hosts. Spread by spores.
Examples: Tinea pedis (athlete's foot in humans), Candida albicans (thrush), Puccinia graminis (wheat stem rust, a major plant pathogen).

Macroparasites

Structure: Multicellular eukaryotic organisms, often with complex life cycles. Includes helminths (worms) and ectoparasites (fleas, ticks).
Mechanism: Live in or on the host, drawing nutrients and causing tissue damage, blood loss or immune dysfunction.
Examples: Taenia solium (pork tapeworm), Schistosoma mansoni (blood fluke causing schistosomiasis), Phytophthora infestans (a protist-like macroparasite causing potato blight).

Worked exam answers

Worked example 1 - classify from a clinical scenario (3 marks)

Question. A patient presents with a high fever, cyclical chills and red blood cell destruction after returning from sub-Saharan Africa. A blood smear shows ring-shaped organisms inside red blood cells. Classify the pathogen and justify your answer.

Model answer. The pathogen is a protozoan, specifically Plasmodium falciparum (malaria). It is a single-celled eukaryote with a nucleus, seen here in its intracellular ring stage inside red blood cells, with a parasitic life cycle and vector-borne transmission by the Anopheles mosquito. Because it is eukaryotic, antibiotics are ineffective (they target prokaryotic structures); antimalarial drugs such as artemisinin combination therapy are required.

Marker's note: 1 mark for naming protozoan, 1 mark for the eukaryotic single-cell justification tied to the smear evidence, 1 mark for the vector/life-cycle or treatment consequence. Naming the type without justification caps at 1 mark.

Worked example 2 - distinguish a virus from a bacterium (3 marks)

Question. Explain why a single-stranded RNA virus cannot be classified as a cellular pathogen.

Model answer. A virus consists only of a strand of nucleic acid (here single-stranded RNA) enclosed in a protein capsid, with no plasma membrane, cytoplasm, ribosomes or other cellular machinery. A cellular pathogen such as a bacterium is a complete cell with a much larger DNA genome and the machinery to metabolise and reproduce independently. Because the virus lacks all cellular structures and must use a host cell to replicate, it is classified as non-cellular (acellular).

Marker's note: 1 mark for the virus structure (nucleic acid + capsid, no cell), 1 mark for contrasting with a cell's independent machinery, 1 mark for the conclusion that it must use a host and is therefore non-cellular. Be specific - "single-stranded RNA", not just "single-stranded".

Worked example 3 - cover plants and animals (4 marks)

Question. Describe four infectious diseases caused by different pathogen types, ensuring you include diseases of both plants and animals.

Model answer. Influenza is caused by influenza A, an enveloped RNA virus, infecting the human respiratory tract (animal). Tuberculosis is caused by Mycobacterium tuberculosis, a bacterium (prokaryotic cell) infecting the lungs (animal). Wheat stem rust is caused by Puccinia graminis, a fungus that spreads by spores and destroys cereal crops (plant). Tobacco mosaic disease is caused by tobacco mosaic virus, a plant virus that mottles tobacco and tomato leaves (plant).

Marker's note: 1 mark per correct disease-pathogen-type triple, with the cap that full marks require at least one plant and one animal example. A list of only human diseases cannot reach 4 marks.

Exam technique

Read the command word. "Classify" or "identify the type" wants the pathogen group plus a justification by structure; "distinguish" or "compare" wants the SAME feature stated for both organisms as a contrast, not two separate lists.

Always justify by a structure the organism has, not by what it does. NESA marker feedback repeatedly says classify by structures (nucleic acid, cell wall, nucleus), and to be specific (single-stranded RNA, not just "single-stranded").

Bank a plant example. Tobacco mosaic virus, wheat stem rust (Puccinia graminis) or crown gall (Agrobacterium tumefaciens) - the dot point explicitly requires plant pathogens, and many candidates lose marks by giving only human diseases.

For a "how would you test if it is viral or bacterial?" item, state the expected result, not just the method: bacteria form colonies on agar and are visible under a light microscope; a virus does neither and is unaffected by antibiotics.

Examples in context

Example 1. Phytophthora cinnamomi causing dieback in Western Australian jarrah forests. Phytophthora cinnamomi is a water mould (oomycete), neither true fungus nor bacterium, that causes dieback in over 40 percent of jarrah (Eucalyptus marginata) forests in southwestern Australia. The pathogen reproduces by motile zoospores that swim through soil water to root tips, where they encyst and germinate, then their hyphae invade and destroy the xylem so the tree cannot transport water. Parks and Wildlife Service WA classifies dieback as the single greatest threat to native flora biodiversity. Quarantine measures including boot-washing stations and vehicle hygiene at park entrances aim to limit pathogen spread, which illustrates how recognising the pathogen type (a water-borne stramenopile) directly shapes control strategy.

Example 2. Hendra virus from flying foxes to horses in NSW. Hendra virus is a paramyxovirus (RNA virus, enveloped) that naturally infects Australian black and grey-headed flying foxes (Pteropus species) without causing disease. Periodically, viral particles in flying fox urine or partially eaten fruit contaminate horse paddocks, infecting horses through ingestion or mucous membrane contact. Affected horses develop fatal respiratory and neurological disease within days. Since 1994, 80 horses and four humans have died in NSW and Queensland outbreaks. Because Hendra is an RNA virus with a lipid envelope, antiviral options are limited; instead control depends on a horse vaccine (Equivac HeV, available since 2012) and removing horses' access to flying fox roosting trees.

Note

Think of pathogens as a line-up of troublemakers sorted from tiny to huge. A prion is the smallest - just a protein folded the wrong way, like a single bent piece of Lego that makes its neighbours bend wrong too. A virus is a bit bigger but still not really alive: it is just instructions in a shell that needs to break into one of your cells to make copies, like a USB stick that does nothing until you plug it into a computer. From there they get properly alive: a bacterium is a single self-running cell, a protozoan is a bigger, fancier single cell (this is what causes malaria), a fungus grows as threads and spreads by spores (like mould or athlete's foot), and a macroparasite is a whole little animal such as a worm or a mite. The smart move in the exam is to tell them apart by what they are built from - not by how big they are - and to remember plants get sick too.

Exam-style practice questions

Practice questions written in the style of NESA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

2025 HSC2 marksThe Varroa mite is an external parasite of European honey bees and is considered to be the most serious pest of honey bees worldwide. Why is Varroa mite infection considered to be an infectious disease?

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Full marks (2) require justifying that Varroa mite infection is infectious. The marking guidelines accept: it is caused by a pathogen (the mite is a parasitic pathogen) that is transmitted/spread between organisms — spreading between bees and hives that come into direct contact with the pest. One mark is given for some relevant information (e.g. naming it a parasite without linking to transmission). The key, flagged in marker feedback, is to identify both that a pathogen causes it AND that it spreads from organism to organism. Source: NESA 2025 HSC Biology examination and marking guidelines.

2023 HSC2 marksDescribe a feature that distinguishes a viral from a bacterial pathogen.

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2 marks: provide one feature that distinguishes a bacterium from a virus. The sample answer compares size — a bacterium is 1–10 µm whereas a virus is 0.05–0.1 µm, so a bacterium is much larger. Other accepted distinguishing features: bacteria are prokaryotic cellular organisms while viruses are non-cellular (nucleic acid + protein coat); bacteria are living while viruses are usually considered non-living; bacteria reproduce by binary fission whereas viruses must use a host cell to replicate. Marker note: state a distinguishing feature as a comparison, not just a list of separate features. Source: NESA 2023 HSC Biology examination and marking guidelines.

2023 HSC2 marksA waterborne disease outbreak occurred after a flood. Outline an experimental procedure that could be used to determine if the pathogen is viral or bacterial.

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2 marks for a method that distinguishes bacterial from viral pathogens including the expected result. Sample answer: try to grow the pathogen on an agar culture — a bacterial pathogen will form colonies on agar, but a viral pathogen cannot be cultured on agar (it needs host cells). Alternatively, view under a high-powered light microscope — bacteria are visible, a virus is too small to see. Another accepted method: expose the pathogen to antibiotics — bacteria die but the virus is unaffected. Marker feedback stresses you must state the expected results, not just the procedure. Source: NESA 2023 HSC Biology examination and marking guidelines.

2020 HSC3 marksThe rabies virus is a single-stranded RNA virus that contains and codes for only five proteins. Use the information provided in a diagram of its structure to explain why the rabies virus cannot be classified as a cellular pathogen.

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3 marks: explain a feature distinguishing the virus from cellular pathogens. Sample answer: the rabies virus has a small genome of single-stranded RNA, whereas cellular pathogens such as bacteria have a much larger genome in the form of DNA, which enables them to carry out complex cellular processes without a host. The virus lacks the cellular machinery to reproduce independently. Marker feedback: be specific (single-stranded RNA, not just "single-stranded") and classify by structures the organism has, not by the processes it carries out. Source: NESA 2020 HSC Biology examination and marking guidelines.

Practice questions

Original practice questions graded from foundation to exam level, each with a full worked solution. Try them before revealing the solution.

foundation2 marksDistinguish between a cellular and a non-cellular pathogen, giving one example of each.

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1 mark - cellular pathogen. A cellular pathogen is made of one or more cells with their own membrane, cytoplasm and machinery to replicate (for example, a bacterium such as Mycobacterium tuberculosis, a protozoan, a fungus or a macroparasite).

1 mark - non-cellular pathogen. A non-cellular (acellular) pathogen has no cell structure and cannot replicate on its own (for example, a virus - nucleic acid in a protein capsid - or a prion, a misfolded protein).

The mark hinges on the contrast: cellular = built from cells, non-cellular = no cell structure. Naming examples without the structural distinction caps at 1 mark.

foundation3 marksName the six categories of pathogen and state one defining structural feature of each.

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Award 1 mark for naming all six, then up to 2 marks for accurate defining features.

Naming (1 mark). Prions, viruses, bacteria, protozoa, fungi, macroparasites.

Defining features (2 marks for the set). Prion - a misfolded protein, no nucleic acid; virus - nucleic acid (DNA or RNA) in a protein capsid, non-cellular; bacterium - prokaryotic single cell with a peptidoglycan cell wall, no nucleus; protozoan - single-celled eukaryote with a nucleus and membrane-bound organelles; fungus - eukaryote (yeast or hyphal mould) with a chitin cell wall; macroparasite - multicellular eukaryote (worm or ectoparasite).

A feature that does not actually distinguish the group (e.g. "small" for a virus) does not earn the mark.

foundation2 marksWhy are prions and viruses described as non-living, while bacteria are described as living?

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1 mark - prions/viruses. Prions (misfolded proteins) and viruses (nucleic acid plus a protein coat) have no cellular structure and cannot carry out metabolism or reproduce independently - a virus must hijack a host cell, so they are usually classed as non-living.

1 mark - bacteria. A bacterium is a complete cell that metabolises and reproduces on its own (by binary fission), so it satisfies the criteria for a living organism.

The discriminator is independent metabolism/reproduction, not size.

core4 marksA patient returning from sub-Saharan Africa has cyclical fevers; a blood smear shows ring-shaped organisms inside red blood cells. Classify the pathogen and justify your classification, then explain why antibiotics would be ineffective.

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1 mark - classification: The pathogen is a protozoan (Plasmodium species, causing malaria).
1 mark - structural justification: Ring-shaped organisms inside red blood cells are single-celled eukaryotes with a nucleus and organelles, living an intracellular parasitic stage - characteristic of a protozoan, not a bacterium (which would not show this red-cell ring stage) or a virus (too small to see on a smear).
1 mark - life-cycle/vector cue: The travel history and cyclical fever fit a vector-borne protozoan transmitted by the Anopheles mosquito.
1 mark - why antibiotics fail: Antibiotics target prokaryotic (bacterial) structures such as peptidoglycan cell walls and 70S ribosomes; a protozoan is eukaryotic and lacks these targets, so antimalarial drugs are required instead.

Linking "eukaryotic - no bacterial target" is what earns the final mark; "antibiotics just don't work" without the reason does not.

core5 marksCompare a bacterium and a virus in terms of structure, classification as cellular or non-cellular, and mode of reproduction, and explain how these differences affect treatment.

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Award up to 5 marks for a genuine comparison (the SAME features for both) plus the treatment link.

Structure (1-2 marks): A bacterium is a prokaryotic cell: peptidoglycan cell wall, plasma membrane, cytoplasm, 70S ribosomes and circular DNA, often with plasmids, but no nucleus. A virus is a non-cellular particle: a strand of nucleic acid (DNA or RNA) inside a protein capsid, sometimes with a lipid envelope, and no cytoplasm or ribosomes.
Cellular vs non-cellular (1 mark): The bacterium is cellular and living; the virus is non-cellular (acellular) and usually considered non-living.
Reproduction (1 mark): The bacterium reproduces independently by binary fission; the virus cannot replicate alone and must inject its genome into a host cell and hijack the host's machinery.
Treatment link (1 mark): Antibiotics disrupt bacterial-specific structures (cell wall, 70S ribosomes), so they kill bacteria but have no effect on viruses, which lack those targets; viral infections need antivirals or vaccines.

Full marks need the comparison framed feature-by-feature for both organisms, not two separate descriptions.

core4 marksNESA requires examples of pathogens in BOTH plants and animals. For three different pathogen types, give one named plant disease or one named animal disease, and identify the pathogen type.

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Award 1 mark for the requirement understanding (plant AND animal examples named) plus up to 3 marks for correct, specific named examples tied to a pathogen type. Any three correct rows score full marks.

Virus: Tobacco mosaic virus - a plant pathogen (mottling of tobacco and tomato leaves); or influenza A - an animal/human viral pathogen.
Bacterium: Agrobacterium tumefaciens - crown gall in plants; or Mycobacterium tuberculosis - tuberculosis in animals/humans.
Fungus: Puccinia graminis - wheat stem rust in plants; or Tinea (Trichophyton) - athlete's foot in animals/humans.

A response with only animal examples cannot reach full marks - NESA explicitly requires a plant example.

exam7 marksTwo unknown pathogens cause outbreaks. Evaluate how knowing the type of each pathogen would shape the strategies used to diagnose, treat and control the resulting diseases. Support your answer with named examples.

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"Evaluate" requires a judgement that weighs how useful pathogen classification is for managing disease, supported by examples. A Band 6 response reaches a reasoned conclusion, not just a list.

Diagnosis (2 marks): The pathogen type dictates the test: a bacterium grows as colonies on agar and is visible under a light microscope; a virus cannot be cultured on agar and needs host cells or molecular tests (e.g. PCR); a protozoan such as Plasmodium is seen on a blood smear; a fungus shows hyphae or spores. Knowing the type tells you which diagnostic to choose.
Treatment (2 marks): Treatment is type-specific: antibiotics work on bacteria (target peptidoglycan, 70S ribosomes) but not on viruses, protozoa or fungi; antivirals/vaccines are needed for viruses (e.g. Equivac HeV for Hendra); antimalarials for the malaria protozoan; antifungals for fungi; and there is no cure for prion disease, only prevention. Misclassifying the pathogen wastes treatment (e.g. antibiotics for a viral cold).
Control (1-2 marks): Control follows transmission, which depends on type: vector-borne protozoa (malaria) are controlled by mosquito control; spore-spread plant fungi (Phytophthora dieback) by quarantine and boot-washing; airborne viruses by isolation and vaccination.
Judgement (1 mark): A supported conclusion: correctly classifying the pathogen is the decisive first step because every later choice - diagnostic test, drug class, and control strategy - flows from it; misclassification leads to wasted treatment and uncontrolled spread. An answer that lists features without an explicit judgement caps below full marks.

exam6 marksA new fatal neurological disease in cattle shows no immune response, no detectable nucleic acid, and is not destroyed by normal sterilisation. Explain why a prion is the most likely cause, and discuss why prion diseases are so difficult to treat and prevent.

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Target a sequenced response that uses the evidence to identify a prion, then explains the treatment/prevention difficulty.

Identify the prion from the evidence (2-3 marks): No detectable nucleic acid rules out viruses, bacteria, protozoa, fungi and macroparasites, which all contain DNA or RNA; a prion is purely a misfolded protein with no nucleic acid, fitting this evidence. No immune response fits a prion because it is a version of the body's own normal protein (PrP), so it is not recognised as foreign. Resistance to normal sterilisation fits the unusually stable, protease-resistant prion aggregate (cf. BSE/"mad cow", and variant CJD in humans).
Mechanism (1 mark): A prion induces normal cellular PrP proteins to misfold into the same abnormal shape, and these aggregates accumulate and destroy brain tissue, producing the spongy, fatal neurodegeneration.
Why hard to treat/prevent (2 marks): There is no nucleic acid to target (so antivirals/antibiotics are useless), no immune response to boost with a vaccine, and the agent resists heat, radiation and standard disinfection, so contaminated instruments or tissue stay infectious. Prevention relies on removing infected tissue from the food chain and destroying contaminated surgical equipment, not on a cure.

Full marks need the evidence-to-prion reasoning, the misfolding mechanism, AND at least two valid reasons treatment/prevention is so hard.