Modes of disease transmission: HSC Biology Module 7

2 marks for an adequate outline of a transmission route (not just naming it). For example, droplet/airborne transmission: when an infected person coughs or sneezes, pathogens are expelled in tiny droplets that are inhaled by another person, allowing the pathogen to enter the new host's respiratory tract. Other acceptable routes outlined with a mechanism: direct contact, contaminated food/water (faecal–oral), bodily fluids, or via a vector. Marker feedback: give an actual outline of how the route works, not merely the name of the mode. Source: NESA 2022 HSC Biology examination and marking guidelines.

3 marks: explain how different modes of transmission affect spread, with more than one example. Sample answer points:

• Diseases spread by airborne droplets (e.g. influenza virus) pass easily between people, so infection rates are high in densely populated areas.
• Diseases requiring an intermediate host or vector spread more slowly, and only where that vector is present.

So direct modes (contact/droplet) generally allow faster, wider spread than indirect/vector-borne modes. Marker feedback: the question says "diseases" — give more than one disease example and contrast the modes.

Source: NESA 2019 HSC Biology examination and marking guidelines.

1 mark - mode of transmission. The mode of transmission is the route or mechanism by which a pathogen moves from one host (or source) to a new host.

1 mark - the distinction. In direct transmission the pathogen passes straight from infected host to new host with no intermediate (e.g. touch, droplet over less than 1 m, sexual contact). In indirect transmission the pathogen reaches the new host via an intermediate - the air, a contaminated vehicle (water, food, fomite) or a living vector.

The mark hinges on the contrast: direct = no intermediary; indirect = an intermediary (air, vehicle or vector) carries the pathogen.

1 mark - airborne

Tuberculosis, caused by Mycobacterium tuberculosis (also accept measles / measles morbillivirus, or influenza A).

1 mark - waterborne

Cholera, caused by Vibrio cholerae (also accept giardiasis / Giardia lamblia).

1 mark - vector-borne

Malaria, caused by Plasmodium falciparum, spread by the Anopheles mosquito (also accept dengue / dengue virus via Aedes aegypti).

Each mark needs a disease correctly matched to its mode; a pathogen named for the wrong mode (e.g. cholera listed as airborne) does not earn the mark.

1 mark - shedding

An infected host sheds the pathogen in faeces.

1 mark - contamination

The faeces contaminate water supplies or food (e.g. through poor sanitation, sewage in drinking water, or unwashed hands preparing food).

1 mark - ingestion

A new host ingests the contaminated water or food, taking the pathogen into the gut where it establishes infection.

The cycle explains why clean water, sanitation and food hygiene are the key controls: each one breaks a different step (shedding to contamination to ingestion). Answers that omit the ingestion step or treat it as airborne cap below full marks.

1 mark - droplet (direct)

Large respiratory droplets (greater than 5 micrometres) are expelled by coughing, sneezing or talking; they are heavy, fall within about 1 metre, and are classed as direct transmission because close contact is needed.

1 mark - airborne

True airborne transmission uses small aerosol particles (less than 5 micrometres) that stay suspended for hours and travel many metres, so a susceptible person can be infected without close contact with the index case.

1 mark - measles behaviour

Measles is genuinely airborne and extremely contagious (R0 of about 12 to 18), so it spreads to many susceptible contacts in a shared space and can infect people who enter a room after the patient has left.

1 mark - containment implication

Because distance and short contact do not stop it, controlling measles needs high vaccination coverage (herd immunity) and airborne precautions (negative-pressure isolation, respirators), not just the physical distancing that limits droplet spread.

Full marks require the 5 micrometre / distance contrast AND linking measles' airborne nature and high R0 to the harder control task.

Award up to 5 marks for a clear cause-and-effect explanation that links each mode to a control that interrupts THAT route, using two contrasting modes.

Principle (1 mark)

Control works by breaking the chain of infection at the transmission step, so the strategy must match the route the pathogen actually uses.

Mode 1 - vector-borne, e.g. malaria (2 marks)

The pathogen reaches new hosts via the Anopheles mosquito. Because the route is the vector, control targets the vector: insecticide-treated bed nets, indoor residual spraying, removing standing water that mosquitoes breed in, and personal repellent. Vaccinating people alone would not remove the vector reservoir.

Mode 2 - airborne/droplet, e.g. influenza or measles (2 marks)

The pathogen passes person-to-person through the air, so control targets person-to-person spread: vaccination to build immunity, masks/respirators, ventilation, and isolation of cases. Vector control is irrelevant here because no vector is involved.

A top answer explicitly states that the same control would fail against the other mode (bed nets do nothing for measles; masks do nothing for malaria), proving the strategy is dictated by the transmission route.

1 mark - attack rate calculation

Attack rate $= \tfrac{320}{500} = 0.64 = 64\%$.

1 mark - identify the mode

Food-borne (indirect) transmission via contaminated catering.

1 mark - justify from incubation

The very short incubation (within 24 hours) points to a food-borne agent such as Staphylococcus aureus toxin or norovirus, rather than a slow vector-borne or person-to-person spread.

1 mark - justify from the common-source pattern

A high attack rate striking many people at once from a single shared event is the classic signature of a common-source (point-source) outbreak, consistent with one contaminated batch of food rather than chains of person-to-person contact.

Both the calculation and a data-anchored justification are needed; naming "food poisoning" without using the incubation and common-source evidence caps at 2-3 marks.

"Evaluate" needs a judgement weighing the strategies against each transmission route, plus a reasoned comparison of spread. A Band 6 answer reaches an explicit, justified conclusion.

Respiratory virus - route and controls (2 marks)

Spread is airborne/droplet, person-to-person, so controls target that route: vaccination, masks/respirators, ventilation, isolation of cases, contact tracing and physical distancing. Strengths: scalable and not weather-dependent. Limitations: hard to sustain, depends on public compliance and rapid vaccine availability; a high R0 means even small gaps allow spread.

Mosquito-borne virus - route and controls (2 marks)

Spread needs the Aedes/Anopheles vector, so controls target the vector: insecticide-treated nets, indoor residual spraying, larval-source reduction (removing standing water), and repellent. Strengths: attacks the obligatory step in the cycle. Limitations: weather/season-dependent (floods and warmth boost breeding), labour-intensive, and ineffective if an animal reservoir exists; vaccinating people may not be available.

Which spreads faster and wider (2 marks)

The respiratory virus is likely to spread faster and wider: airborne person-to-person transmission needs only shared air and proceeds independent of weather or a vector, giving a higher effective reproduction number, whereas the mosquito-borne virus is rate-limited by vector abundance, biting behaviour and season, confining spread to where and when the vector is active.

Judgement (1 mark)

A supported conclusion: prioritise person-to-person controls (vaccination, masks, ventilation, isolation) for the faster respiratory threat, while running vector control for the mosquito-borne virus, because each strategy only works against the route its pathogen uses. An answer that lists strategies without weighing them or without an explicit faster/wider judgement caps below full marks.

Target a sequenced response that links each outbreak's mode to its specific controls, then explains why the strategies are not interchangeable.

Outbreak 1 - direct contact, e.g. West African Ebola 2014-2016 (2 marks)

Ebola spread by direct contact with infected bodily fluids, including during traditional burials. Because the route was person-to-person contact, control interrupted contact: case isolation, contact tracing, personal protective equipment, modified safe burials and ring vaccination.

Outbreak 2 - waterborne, e.g. Sydney Cryptosporidium/Giardia 1998 (2 marks)

Contamination of the drinking-water supply meant a faecal-oral, waterborne route. Control therefore targeted the water vehicle: "boil water" alerts, catchment monitoring, and upgrading filtration (ultrafiltration/UV) because the oocysts are chlorine-resistant.

Why strategies are not interchangeable (2 marks)

Each control breaks the chain only at the route actually used: boiling water and filtration do nothing to stop Ebola passing between people, and isolating cases or modifying burials does nothing to decontaminate a city's reservoir. Because the transmission vehicle differs (human contact vs contaminated water), the intervention point differs, so the response must be tailored to the mode.

Full marks need both outbreaks correctly classified, mode-matched controls for each, AND an explicit argument that the controls are route-specific and not transferable.