← Module 7: Infectious Disease
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.
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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.
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 example
A patient presents with a high fever, cyclical chills and red blood cell destruction after returning from sub-Saharan Africa. Blood smear shows ring-shaped organisms inside red blood cells.
Classification. The pathogen is a protozoan, specifically Plasmodium falciparum.
Justification. Single-celled eukaryote, parasitic life cycle inside red blood cells, transmitted by a vector (Anopheles mosquito). Antibiotics are ineffective because the pathogen is eukaryotic. Antimalarial drugs (artemisinin combination therapy) are required.
Common traps
Calling viruses "living." They are acellular and cannot replicate independently. Most syllabus answers describe them as "non-cellular" or "acellular particles."
Forgetting plant pathogens. NESA requires examples in plants and animals. Include at least one plant pathogen (tobacco mosaic virus, wheat stem rust, or crown gall).
Mixing protozoa with bacteria. Protozoa are eukaryotic single-celled organisms with a nucleus and membrane-bound organelles. Bacteria are prokaryotic.
Generic disease names. "A virus" or "a bacterium" scores no marks. Use full scientific names where possible.
In one sentence
Infectious diseases are caused by six pathogen types (prions, viruses, bacteria, protozoa, fungi and macroparasites), each with a defining structure and characteristic mode of replication that determines how the disease is treated.
Past exam questions, worked
Real questions from past NESA papers on this dot point, with our answer explainer.
2021 HSC4 marksCompare the structure of viruses and bacteria, and explain why this difference affects treatment options.Show worked answer →
A 4-mark answer needs two structural comparisons and a clear treatment link.
Structure. Bacteria are prokaryotic single-celled organisms with a cell wall, plasma membrane, cytoplasm, ribosomes and circular DNA. They are living and reproduce independently by binary fission. Viruses are acellular particles consisting of nucleic acid (DNA or RNA) enclosed in a protein coat (capsid), sometimes with a lipid envelope. They are not considered living and cannot reproduce without a host cell.
Size. Bacteria are typically 1 to 10 micrometres. Viruses are 20 to 300 nanometres, around 100 times smaller.
Treatment. Antibiotics target bacterial-specific structures such as the cell wall (penicillin) or 70S ribosomes (tetracycline). Viruses lack these structures, so antibiotics are ineffective. Viral infections are treated with antivirals (e.g. oseltamivir for influenza) that block viral replication enzymes, or prevented with vaccines.
Markers reward naming at least one antibiotic and one antiviral, and stating explicitly that antibiotics target bacterial structures that viruses lack.
2019 HSC3 marksIdentify three different types of pathogen and provide a named disease caused by each.Show worked answer →
A 3-mark answer needs three correctly classified pathogens with a specific named disease.
- Bacteria. Mycobacterium tuberculosis causes tuberculosis, a respiratory infection that damages lung tissue.
- Virus. Influenza A virus causes seasonal influenza, infecting the respiratory tract.
- Protozoan. Plasmodium falciparum causes malaria, transmitted by the Anopheles mosquito.
Other valid examples include prions (BSE), fungi (Candida albicans causing thrush) and macroparasites (Taenia solium, the pork tapeworm). Markers reward correctly matching the pathogen type to the disease and using full scientific names where possible.
Related dot points
- Investigate the transmission of a disease during an epidemic, including: mode of transmission (direct, indirect including airborne, vector-borne and waterborne or food-borne) of an infectious disease
A focused answer to the HSC Biology Module 7 dot point on modes of transmission. Covers direct transmission, indirect transmission (airborne, waterborne, food-borne) and vector-borne transmission, with a named example for each and the public-health implications.
- Investigate the transmission of a disease during an epidemic, including: adaptations of pathogens that facilitate their entry into and transmission between hosts
A focused answer to the HSC Biology Module 7 dot point on pathogen adaptations. Covers structural and biochemical adaptations that allow entry into hosts, evasion of immune responses and transmission between hosts, with named examples for each.
- Investigate the work of Pasteur and Koch and evaluate the impact of their work on the understanding of infectious disease, including Koch's postulates
A focused answer to the HSC Biology Module 7 dot point on Pasteur and Koch. Covers Pasteur's swan-neck flask experiment, Koch's anthrax and tuberculosis work, the four Koch's postulates, and the impact of germ theory on modern medicine.