← Module 7: Infectious Disease
Inquiry Question 1: How are diseases transmitted?
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.
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What this dot point is asking
NESA wants you to classify the main modes of transmission, give a named example of a disease for each mode, and explain how the mode determines public health responses. Transmission mode comes up in multiple choice every year and is central to extended-response questions on epidemics.
The answer
Transmission is the process by which a pathogen moves from one host to another. The four main modes are direct, airborne, waterborne or food-borne, and vector-borne. The first is "direct"; the rest are forms of indirect transmission.
Direct transmission
The pathogen passes from infected host to new host through physical contact, with no intermediate.
Routes. Touch (skin, mucous membranes), sexual contact, mother-to-child during birth or breastfeeding, droplet spread over short distances (less than 1 metre).
Examples. HIV (sexual contact, blood-to-blood), glandular fever caused by Epstein-Barr virus (saliva), and tinea (skin-to-skin or shared towels).
Indirect transmission: airborne
The pathogen travels through the air on aerosol droplets or dust particles, sometimes over long distances.
Mechanism. Coughing, sneezing or talking produces aerosolised droplets. Smaller droplets (less than 5 micrometres) can remain suspended for hours and travel many metres.
Examples. Mycobacterium tuberculosis (tuberculosis), influenza A, SARS-CoV-2 (COVID-19), measles morbillivirus. Measles is one of the most contagious airborne pathogens, with an R0 of 12 to 18.
Indirect transmission: waterborne and food-borne
The pathogen is carried in contaminated water or food.
Mechanism. Faecal-oral cycle is the most common pattern. An infected host sheds the pathogen in faeces, which contaminates water supplies or food. A new host ingests the pathogen.
Examples. Vibrio cholerae (cholera, contaminated water), Salmonella enterica (food poisoning, undercooked poultry and eggs), hepatitis A virus (contaminated shellfish), Giardia lamblia (contaminated water).
Indirect transmission: vector-borne
A living organism, the vector, carries the pathogen between hosts. The vector is usually an arthropod (mosquito, tick, flea).
Mechanism. The vector picks up the pathogen from one host's blood, the pathogen may undergo development inside the vector, and the vector then transfers the pathogen to a new host through bites or faeces.
Examples. Plasmodium falciparum (malaria, Anopheles mosquito), Yersinia pestis (plague, fleas on rodents), dengue virus (Aedes aegypti mosquito), Trypanosoma brucei (African sleeping sickness, tsetse fly).
Plant pathogens
The same modes apply in plants, with some plant-specific routes such as transmission via grafting and by aphid vectors (e.g. tobacco mosaic virus).
Worked example
During the 2014 to 2016 West African Ebola outbreak, the virus spread primarily by direct contact with bodily fluids of infected patients, including during traditional burial practices.
Classification. Direct transmission.
Implication for control. Strategies that interrupt person-to-person contact were the most effective: case isolation, contact tracing, personal protective equipment for healthcare workers, modified burial practices, and ring vaccination.
Common traps
Confusing airborne and droplet transmission. Droplets larger than 5 micrometres fall within a metre and are classed as direct contact. True airborne pathogens (tuberculosis, measles) travel further on smaller particles.
Forgetting that vector-borne transmission requires a living vector. A contaminated needle is not a vector. Vectors are biological organisms, often with the pathogen undergoing part of its life cycle inside them.
Generic descriptions. "It spreads through the air" is not enough. Specify droplets, aerosols, or contaminated dust, and give the typical distance and duration.
Ignoring food-borne illness. Many students forget that food can be a transmission vehicle, not just water. Salmonella, E. coli O157, and listeria are common food-borne pathogens.
In one sentence
Infectious diseases are transmitted directly (touch, droplet, sexual contact, vertical) or indirectly (airborne, waterborne or food-borne, vector-borne), and the transmission mode determines which public health strategies will most effectively interrupt the chain of infection.
Past exam questions, worked
Real questions from past NESA papers on this dot point, with our answer explainer.
2022 HSC5 marksUsing a named example, describe how an infectious disease is transmitted and outline two strategies that interrupt transmission.Show worked answer →
A 5-mark answer needs a named pathogen, a clear transmission mode and two specific control strategies.
Named example. Cholera is caused by the bacterium Vibrio cholerae.
Mode of transmission. Indirect, waterborne. The pathogen is shed in the faeces of infected people and contaminates drinking water. Healthy individuals become infected when they drink contaminated water or eat food washed in it. The bacterium colonises the small intestine and releases cholera toxin, causing severe watery diarrhoea, which further contaminates the water supply.
Strategy 1. Sanitation infrastructure. Treating sewage and chlorinating drinking water breaks the faecal-oral cycle, removing the pathogen before it reaches new hosts. This was the basis of John Snow's 1854 Broad Street pump intervention in London.
Strategy 2. Oral rehydration therapy and oral cholera vaccines. While rehydration treats infected patients, vaccines (e.g. Dukoral) provide immunity and reduce shedding in endemic regions, lowering the reservoir of pathogen.
Markers reward naming the pathogen, the specific transmission route, and linking each strategy to the mechanism it interrupts.
Related dot points
- 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.
- 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 and assess the effectiveness of historical and contemporary methods of prevention and control of infectious disease, including local, regional and global strategies (hygiene, quarantine, vaccination and public health campaigns)
A focused answer to the HSC Biology Module 7 dot point on disease control strategies. Covers hygiene, quarantine, vaccination programs, public health campaigns, and the role of the WHO, with named examples at each scale and a frank assessment of effectiveness.