Inquiry Question 1: How are diseases transmitted?
Investigate the transmission of a disease during an epidemic, including: adaptations of pathogens that facilitate their entry into and transmission between hosts
A focused HSC Biology Module 7 answer on pathogen adaptations. Covers structural and biochemical adaptations for entry into hosts, immune evasion and transmission between hosts, with named examples (influenza, malaria, cholera, TB) and worked exam answers.
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What this dot point is asking
NESA wants you to identify the structural, biochemical and behavioural adaptations that pathogens use to enter hosts and pass between them. Strong answers cite named adaptations with their specific function and link them to transmission mode.
The command words range from "outline" and "identify" up to "compare" and "assess", so match your depth to the verb - and always name a specific pathogen and a specific feature (haemagglutinin, urease, mycolic acid, gametocyte), never a vague "it has surface proteins".
The answer
Pathogens have evolved specific adaptations that solve three problems: getting into a new host, evading the host's defences, and getting back out to a new host. These adaptations are usually shaped by the transmission mode.
Adaptations for entry into the host
- Surface attachment proteins
- Many pathogens carry surface molecules that bind to specific host cell receptors. Influenza haemagglutinin binds to sialic acid on respiratory cells. HIV gp120 binds to CD4 receptors on T helper cells. Plasmodium sporozoites bind to hepatocyte surface proteins.
- Enzymes that breach tissue barriers
- Streptococcus pyogenes produces hyaluronidase and streptokinase, which break down connective tissue and clot proteins, allowing the bacterium to spread through skin and soft tissue. Some fungi secrete keratinases to digest skin.
- Specialised entry structures
- Bacteriophages and many bacterial pathogens use pili and fimbriae to attach to host cells before invasion. Salmonella uses a type III secretion system, a needle-like structure, to inject proteins that force gut cells to engulf the bacterium.
- Spore and cyst stages
- Bacillus anthracis forms endospores that resist heat, drying and chemical insult, allowing the pathogen to remain infectious in soil for decades. Giardia lamblia forms tough cysts that survive in water until ingested.
Adaptations for evading the host's defences
- Antigenic variation
- Influenza and HIV mutate rapidly (antigenic drift) so that antibodies raised against earlier strains do not recognise new ones. Trypanosoma brucei changes its surface glycoprotein coat repeatedly, evading antibody recognition.
- Capsules and biofilms
- Streptococcus pneumoniae has a polysaccharide capsule that prevents phagocytosis. Pseudomonas aeruginosa forms biofilms that block antibiotics and immune cells.
- Intracellular hiding
- Viruses replicate inside host cells, hidden from antibodies. Mycobacterium tuberculosis survives inside macrophages, the very cells meant to destroy it.
Adaptations for transmission between hosts
- Inducing symptoms that spread the pathogen
- Vibrio cholerae triggers severe watery diarrhoea, flooding water supplies with new bacteria. Influenza triggers coughing and sneezing, aerosolising the virus. Rabies virus alters host behaviour to encourage biting.
- Vector-specific adaptations
- Plasmodium has separate stages for the mosquito and human host, with surface proteins matching each. The parasite manipulates mosquito feeding behaviour to favour transmission.
- Environmental durability
- Norovirus is non-enveloped and resists drying, surviving on surfaces for weeks. Prions resist boiling, UV and standard disinfection, allowing transmission via contaminated surgical instruments.
- High shedding rate
- Measles virus produces enormous numbers of virions in the airway, and an infected person typically infects 12 to 18 susceptibles in a fully susceptible population.
Examples in context
Example 1. Influenza antigenic drift and the annual vaccine reformulation. Influenza A virus carries two surface glycoproteins, haemagglutinin (HA) and neuraminidase (NA), which are the major targets of host antibodies. The viral RNA polymerase lacks proofreading, so mutations accumulate continuously in HA and NA (antigenic drift). The Australian Influenza Vaccine Committee reviews global surveillance data from WHO sentinel labs in February each year to decide which HA and NA variants to include in the southern hemisphere vaccine, ready for May rollout. Periodically, two influenza strains reassort their RNA segments inside a co-infected pig or bird host (antigenic shift), producing a novel strain such as the 2009 H1N1 pandemic strain that swept Sydney. Both adaptations help influenza evade adaptive immunity.
Example 2. Bacillus anthracis spores and biosecurity at NSW abattoirs. Bacillus anthracis, the cause of anthrax in cattle, produces highly durable endospores when nutrients become scarce. The spores have a thick keratin-like coat that resists boiling, desiccation, UV radiation and many disinfectants for decades. In NSW, sporadic anthrax outbreaks occur in livestock when cattle graze over old burial sites of previously infected animals, sometimes more than 50 years later. NSW DPI biosecurity protocols require carcasses of anthrax-suspected animals to be incinerated and buried at least 2 metres deep, and abattoir surfaces are disinfected with sporicidal agents (formaldehyde or peracetic acid) because routine disinfectants cannot inactivate the spores.
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 marksOutline an adaptation in a pathogen that facilitates transmission between hosts.Show worked answer →
2 marks for outlining a pathogen adaptation that aids transmission to a new host. Sample answer: after colonising a host, fungi can produce spores which are released to infect a new host. The adaptation (spore production/release) directly enables the pathogen to move from one host to another. One mark for some relevant information. Marker feedback (from related years): give an adaptation that aids transmission between hosts, not one that merely benefits the pathogen within the host. Source: NESA 2025 HSC Biology examination and marking guidelines.
2020 HSC2 marksRabies is transmitted by the bite of an infected animal. Using a diagram showing rabies infection (virus enters from saliva, replicates near the bite, travels up nerves to the CNS and brain, then enters the salivary glands), identify TWO features of the rabies infection that facilitate transmission of the pathogen to a new host.Show worked answer →
2 marks for identifying features that facilitate transmission between hosts. Sample answer: the virus is able to travel via the nervous system to the salivary glands, and this can result in direct-contact transmission when the infected host bites another animal (saliva carrying virus enters the new host through the bite wound). The two linked features are: virus reaching the saliva, and the bite delivering it into a new host. Marker feedback: recognise that the bite is essential (saliva alone cannot penetrate intact skin), and don't be distracted by stimulus details about the disease's effect on the original host. Source: NESA 2020 HSC Biology examination and marking guidelines.
2019 HSC2 marksOutline ONE adaptation of a specific pathogen that facilitates its entry into a host.Show worked answer →
2 marks: name a specific pathogen and outline an adaptation that helps it enter the host. Sample answer: the bacterium Helicobacter pylori causes stomach ulcers and has a flagellum that allows it to move and penetrate the mucus lining/barrier of the stomach wall, gaining entry to the tissue. One mark for some relevant information. Marker feedback: name a specific pathogen (not just a disease) and make sure the adaptation is for entry, not for immune evasion or transmission. Source: NESA 2019 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 marksName ONE surface-attachment adaptation of a named pathogen and state the host molecule it binds.Show worked solution →
1 mark - named pathogen + named adaptation. Influenza virus carries the surface glycoprotein haemagglutinin (HA).
1 mark - the host molecule it binds. HA binds sialic acid receptors on the surface of host respiratory epithelial cells, allowing the virus to attach before entry.
The mark hinges on naming both the specific molecule (haemagglutinin, gp120, pili) AND the receptor it binds. "It has proteins on its surface" earns nothing - markers reward the named protein and named receptor.
foundation3 marksOutline how each of the following helps a pathogen succeed in a host: (a) a tissue-degrading enzyme, (b) a polysaccharide capsule, (c) an endospore.Show worked solution →
- 1 mark - tissue-degrading enzyme
- An enzyme such as hyaluronidase (from Streptococcus pyogenes) breaks down connective tissue, letting the bacterium spread through skin and soft tissue (aids entry/spread within the host).
- 1 mark - polysaccharide capsule
- A capsule (e.g. on Streptococcus pneumoniae) prevents phagocytosis, so the pathogen evades the innate immune response (immune evasion).
- 1 mark - endospore
- An endospore (e.g. Bacillus anthracis) is a tough dormant stage that resists heat, drying and disinfectants, keeping the pathogen infectious in the environment for transmission to a new host.
Each adaptation must be tied to its specific job (spread / evade / survive-and-transmit). Naming the structure without its function does not earn the mark.
foundation2 marksDistinguish between an adaptation for ENTRY into a host and an adaptation for TRANSMISSION between hosts, giving one example of each.Show worked solution →
1 mark - entry adaptation. An entry adaptation helps the pathogen get into a new host's tissues - for example Helicobacter pylori's flagellum, which drives it through the stomach mucus to reach the wall.
1 mark - transmission adaptation. A transmission adaptation helps the pathogen move from one host to the next - for example influenza triggering coughing and sneezing that aerosolise the virus to new hosts.
The contrast is the discriminator: entry = crossing into one host; transmission = passing between hosts. Blurring the two caps at 1 mark.
core4 marksExplain how antigenic drift and antigenic shift allow influenza to evade the adaptive immune response, and why this matters for the annual vaccine.Show worked solution →
- 1 mark - antigenic drift mechanism
- Influenza's RNA polymerase lacks proofreading, so point mutations accumulate continuously in the genes for haemagglutinin (HA) and neuraminidase (NA).
- 1 mark - effect of drift
- These small surface changes mean antibodies raised against earlier strains no longer recognise the altered HA/NA, so prior immunity (or last year's vaccine) is less effective.
- 1 mark - antigenic shift mechanism
- When two influenza strains co-infect one host (e.g. a pig or bird), their segmented RNA genomes reassort, producing a novel HA/NA combination (e.g. the 2009 H1N1 strain).
- 1 mark - link to the vaccine
- Because drift constantly changes the antigens, the vaccine must be reformulated each year to match the strains predicted to circulate; shift can produce a strain so novel that a pandemic results.
Full marks need both the mutation mechanism AND the immune-evasion consequence, plus the vaccine link. Distinguish drift (gradual point mutation) from shift (reassortment of whole segments).
core5 marksCompare the adaptations that facilitate transmission between hosts in Plasmodium (malaria) and Vibrio cholerae (cholera).Show worked solution →
Award up to 5 marks for a genuine comparison (named adaptations for both, with similarities and differences) tied to transmission.
- Plasmodium (2 marks)
- Plasmodium has a vector-borne life cycle with distinct stages for the human and mosquito hosts. It produces gametocytes in human blood that are taken up when a female Anopheles mosquito feeds; the parasite completes sexual development in the mosquito and is delivered as sporozoites in saliva when the mosquito next bites. It manipulates mosquito feeding behaviour to favour transmission. Transmission is therefore indirect, via a vector.
- Vibrio cholerae (2 marks)
- V. cholerae secretes cholera toxin, which triggers massive watery diarrhoea. This floods water supplies with huge numbers of bacteria, which infect new hosts when contaminated water is drunk. Transmission is direct, faecal-oral, via the environment - the symptom itself is the transmission adaptation.
- The comparison (1 mark)
- Both use a non-airborne route and both exploit a stage/product that carries the pathogen out of the host, but Plasmodium needs a living vector and a multi-stage life cycle, whereas V. cholerae uses a toxin-driven symptom and contaminated water with no vector.
A response that describes only one organism, or lists features without an explicit comparison, does not reach full marks.
core4 marksA laboratory finds that 95 percent of Helicobacter pylori survive at pH 2 in stomach acid, compared with 2 percent of Escherichia coli. Identify the adaptation responsible, explain its mechanism, and identify a second adaptation that helps H. pylori reach the stomach wall.Show worked solution →
- 1 mark - identify the acid-survival adaptation
- H. pylori produces the enzyme urease.
- 1 mark - mechanism
- Urease converts urea into ammonia, which is alkaline and neutralises the acid in a microenvironment surrounding the bacterium, so it survives pH 2 that kills E. coli.
- 1 mark - second adaptation
- H. pylori has a flagellum (corkscrew motility).
- 1 mark - its function
- The flagellum lets it burrow through the protective mucus layer to reach the less-acidic stomach wall, where it colonises the epithelium (entry into the tissue).
Both the enzyme/ammonia logic and the flagellum/mucus logic are required; naming urease without the ammonia-neutralisation step caps the first part at 1 mark.
exam7 marksCompare the adaptations of TWO named pathogens that facilitate their entry into, and transmission between, hosts. In your answer, assess how the transmission mode shapes the adaptations seen.Show worked solution →
"Compare" requires the same features set side by side for both pathogens; "assess" requires a judgement linking the adaptations back to the transmission mode. Choose two pathogens with contrasting transmission modes - for example influenza (airborne/droplet) and Plasmodium (vector-borne).
- Influenza - entry and transmission (2 marks)
- Entry: haemagglutinin binds sialic acid on respiratory cells, letting the virus attach and be endocytosed. Transmission: high replication in the airway plus coughing and sneezing aerosolise virions; antigenic drift/shift keeps a susceptible population available. The adaptations are tuned to a droplet/airborne route - surface binding to airway cells and aerosol release.
- Plasmodium - entry and transmission (2 marks)
- Entry: sporozoites injected in mosquito saliva bind hepatocyte surface proteins to enter the liver. Transmission: a multi-stage life cycle produces gametocytes taken up by a feeding mosquito, with the vector delivering sporozoites to the next host. The adaptations are tuned to a vector-borne route - stage-specific surface proteins matching each host and behaviour manipulation of the mosquito.
- Comparison and assessment (2-3 marks)
- Both rely on stage- or surface-specific attachment to a particular host cell, but the transmission mode dictates the rest: an airborne pathogen invests in aerosolisation and rapid antigenic change to keep spreading person-to-person, whereas a vector-borne pathogen invests in a complex life cycle and vector manipulation because it cannot survive outside a host or vector. A reasoned judgement - that transmission mode is the primary selective pressure shaping each adaptation set - lifts the response to Band 6.
A response that describes two pathogens without an explicit comparison, or that omits the assessment of transmission mode, caps below full marks.
exam6 marksMycobacterium tuberculosis can persist in human lungs for decades. Explain how its adaptations allow it to evade the immune response and remain transmissible, and discuss one consequence of this for controlling the disease in a population.Show worked solution →
Target a sequenced response linking specific evasion adaptations to persistence and onward transmission, then a population-level consequence.
- Immune-evasion adaptations (2-3 marks)
- M. tuberculosis has a thick, waxy mycolic-acid cell wall that resists phagocytic digestion. It survives and replicates inside macrophages - the very cells meant to destroy it - hidden from antibodies. The host walls it off in a granuloma, inside which bacteria can remain dormant (latent) for years.
- Link to persistence and transmission (1-2 marks)
- Because the bacteria persist latently, an infected person can carry TB for decades; on reactivation the bacteria multiply in the airway and are coughed out as aerosols, transmitting to new hosts long after initial infection.
- Population-level consequence (1-2 marks)
- Latency creates a large reservoir of asymptomatic carriers who are not obviously sick but can reactivate and transmit later. This makes TB hard to eradicate: control requires screening for latent infection, long antibiotic courses, and contact tracing, not just treating symptomatic cases - and incomplete treatment selects for antibiotic-resistant strains.
Full marks need specific named adaptations (mycolic acid, intracellular survival, granuloma/latency), the transmission link, AND a valid population consequence that follows from latency.