How Infectious Disease Spreads

HSC Biology | Study Notes

How infectious disease spreads is a key part of NSW Biology Stage 6, Module 7, Infectious Disease. This topic matters because Module 7 specifically asks students to investigate modes of transmission of infectious diseases, including direct contact, indirect contact and vector transmission, and to investigate the transmission of disease during an epidemic. It also links to BIO12-14, which focuses on analysing infectious disease in terms of cause, transmission, management and the organism’s response. 

In this lesson

• what transmission means in infectious disease

• how direct transmission works

• how indirect transmission works

• what vectors are

• how pathogens spread from one host to another

What is transmission?

Transmission is the way a pathogen moves from one host to another.

This is one of the biggest ideas in Module 7 because the syllabus specifically asks, How are diseases transmitted? and includes direct contact, indirect contact and vector transmission. 

Why transmission matters

The mode of transmission affects:

• how quickly a disease spreads

• which populations are most at risk

• which control strategies are likely to work best

A 2019 HSC marking guideline states that the mode of transmission influences the spread of disease because some diseases spread easily between people, while others depend on vectors or intermediate hosts. 

Direct transmission

Direct transmission happens when a pathogen passes straight from one host to another without an intermediate object or vector.

Common examples of direct transmission

Direct transmission can happen through:

• physical contact

• exchange of body fluids

• droplets passed directly between people at close range

• bites from an infected host

Why direct transmission can spread quickly

Direct transmission can lead to rapid spread when:

• people are in close contact

• populations are dense

• infected body fluids or droplets pass easily between hosts

A 2019 HSC marking guideline gives airborne droplets, such as influenza virus droplets, as an example of a disease spreading easily between people, especially in populated areas. 

HSC-style example

A 2020 HSC marking guideline explains that rabies virus can reach the salivary glands and then be transmitted by direct contact when an infected host bites another animal. 

Indirect transmission

Indirect transmission happens when a pathogen is spread without direct host-to-host contact.

What this can involve

Indirect transmission may involve:

• contaminated food

• contaminated water

• contaminated surfaces or objects

• another stage between infected and uninfected hosts

Why indirect transmission matters

Indirect transmission can spread disease even when infected and uninfected hosts never meet directly.

This is why hygiene and food or water safety are important in disease control. The Module 7 syllabus also includes practical work related to microbial testing of water or food samples, which links directly to indirect transmission. 

Vectors

A vector is an organism that carries a pathogen from one host to another.

What vectors do

Vectors help pathogens spread by:

• moving between hosts

• carrying the pathogen with them

• introducing the pathogen into a new host

Common vector examples

Common vectors include:

• mosquitoes

• flies

• ticks

Why vector transmission matters

If a pathogen depends on a vector, the spread of disease depends on:

• the presence of the vector

• the distribution of the vector

• how often the vector contacts hosts

A 2019 HSC marking guideline states that if an intermediate host or vector is required, the spread of disease may be slower and depends on the presence of the vector. 

HSC-style example

A 2024 HSC question on African sleeping sickness identified:

• the pathogen as trypanosomes

• the vector as the tsetse fly

• the method of transmission to humans as indirect transmission 

A 2019 HSC paper also describes dengue fever and malaria as diseases transmitted between humans by mosquitoes. 

Host-to-host spread

Host-to-host spread means the movement of a pathogen from one infected host to a new host.

How this happens

Host-to-host spread may occur by:

• direct contact

• indirect contact

• vector transmission

Why this matters

The same disease may spread differently depending on the pathogen and its adaptations.

For example:

• a respiratory pathogen may spread rapidly through droplets

• a vector-borne disease may spread only where the vector is present

• a food-borne pathogen may spread through contamination

The Module 7 syllabus also asks students to compare pathogen adaptations that help entry into hosts and transmission between hosts. 

Comparing the main modes of transmission

Mode of transmission	Main idea	Example idea
Direct transmission	Pathogen passes straight from host to host	Rabies spread by bite
Indirect transmission	Pathogen spreads without direct contact	Contaminated food or water
Vector transmission	A living organism carries the pathogen	Tsetse fly carrying trypanosomes

Why the mode of transmission affects disease spread

The mode of transmission changes how easily a disease moves through a population.

Directly transmitted diseases

These may spread quickly in:

• crowded places

• close-contact settings

• populations with frequent person-to-person interaction

Vector-borne diseases

These depend on:

• the distribution of the vector

• environmental conditions suitable for the vector

• how often hosts and vectors meet

A 2019 HSC marking guideline links the global spread of dengue fever and malaria to mosquito vectors, population density and increased travel. 

Worked example

Exam-style question

Explain how the mode of transmission of a pathogen can influence the spread of disease.

Worked answer

The mode of transmission affects how easily a disease spreads. A directly transmitted pathogen, such as one spread in droplets, can move quickly through dense populations because hosts are in close contact. A pathogen that requires a vector may spread more slowly or only in areas where the vector is present. 

Why this works

This answer:

• refers to more than one transmission mode

• explains how spread is affected

• links transmission to population conditions and vectors

Common mistakes

• Treating direct and indirect transmission as the same thing.

• Forgetting that vectors are living organisms.

• Saying all infectious diseases spread directly from person to person.

• Naming a vector without naming the pathogen it carries.

• Describing a disease without explaining how its transmission affects spread.

Quick quiz

1. What is transmission in infectious disease?

2. What is direct transmission?

3. What is indirect transmission?

4. What is a vector?

5. Why can the mode of transmission affect how quickly a disease spreads?

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