Learning Objectives
What you'll learn this week
Week Overview
Core concepts and explanations
This week focuses on infectious diseases — illnesses caused by pathogenic microorganisms such as bacteria, viruses, parasites, and fungi. An important distinction is made between infectious diseases (caused by pathogens) and communicable diseases (those that can spread from person to person). All communicable diseases are infectious, but not all infectious diseases are communicable — for example, tetanus is caused by bacteria but cannot be passed between people. Infectious diseases remain a leading cause of death in low-income countries and are still common everywhere, even if many (like the common cold) cause only mild illness.
The spread of infectious disease is explained through two key models. The epidemiologic triad shows that disease results from the interaction of three factors: an agent (the pathogen), a host (the person who gets sick), and the environment (the conditions that bring them together). The chain of infection breaks this down further into six links: the infectious agent, the reservoir where it lives, the portal of exit from that reservoir, the mode of transmission, the portal of entry into a new host, and the susceptible host. Understanding these links is critical because breaking any single link can prevent infection. Modes of transmission include direct contact, droplet spread, airborne particles, vehicle-borne (through contaminated objects like food or water), and vector-borne (through insects like mosquitoes).
Public health uses several strategies to control infectious diseases. Preventative methods include vaccination, hand hygiene, safe food and water practices, personal protective equipment (PPE), and health education. Sentinel surveillance systems in Australia monitor specific diseases through selected sites — for example, sentinel chickens detect mosquito-borne viruses, and GP networks track influenza-like illness. Some diseases are classified as quarantinable (such as cholera, plague, rabies, SARS, and Ebola) because they pose extreme public health risks. Contact tracing identifies people who have been exposed to an infectious case and manages them to break chains of transmission. In Australia, over 1.5 million cases of notifiable diseases were reported in 2023, with COVID-19 accounting for more than half of all notifications.
Topic 7 provides a comprehensive examination of infectious disease epidemiology, transmission dynamics, and public health control strategies. The foundational distinction between infectious and communicable diseases is established: infectious diseases are caused by pathogenic microorganisms (bacteria, viruses, parasites, fungi, prions), while communicable diseases specifically denote person-to-person transmissibility. This distinction has practical implications — diseases like tetanus are infectious but non-communicable, requiring different control strategies than communicable pathogens. Zoonotic diseases, transmissible from vertebrate animals to humans under natural conditions, represent another important category encompassing diseases such as rabies, plague, and avian influenza.
Disease transmission is conceptualised through two complementary models. The epidemiologic triad posits that disease results from the interaction of agent (pathogen, characterised by virulence, pathogenicity, and dose), host (influenced by genetic composition, immunological status, nutritional state, age, and behavioural factors), and environment (encompassing physical, biological, and socioeconomic factors including sanitation, crowding, and health service availability). The chain of infection extends this framework into a sequential model: infectious agent → reservoir (human, animal, or environmental) → portal of exit → mode of transmission (direct contact, droplet spread, airborne, vehicle-borne, or vector-borne) → portal of entry → susceptible host. The natural history of disease describes progression from exposure through the incubation period (time to symptom onset), the latent period (time to infectiousness), the period of communicability, and ultimately to recovery, disability, or death. The concepts of carriers (infected individuals capable of transmitting pathogens without exhibiting symptoms) and the spectrum of disease (ranging from subclinical to fatal) are essential for understanding why clinical cases represent only the 'tip of the iceberg' of actual infections.
Disease occurrence patterns are classified as endemic (constantly present at predictable levels), epidemic (sudden increase above expected levels, which may be point-source or extended-source), or pandemic (epidemic spread across multiple countries or continents). Australia employs a multi-layered infectious disease control infrastructure including sentinel surveillance programs (sentinel chickens for arboviral monitoring, ASPREN for community illness tracking, ACIR for immunisation coverage, OzFoodNet for foodborne illness), management of quarantinable diseases (cholera, avian influenza, plague, rabies, SARS, smallpox, viral haemorrhagic fevers, yellow fever), contact tracing conducted by state and territory public health units, and isolation protocols for confirmed cases. Vaccination programs serve to prevent disease, protect vulnerable populations, achieve herd immunity, reduce healthcare costs, and potentially eradicate diseases. National surveillance operates through the NNDSS, with over 1.5 million notifiable disease cases reported in 2023, predominantly COVID-19 (54%), followed by laboratory-confirmed influenza, RSV, chlamydia, campylobacter, and gonorrhoea. Non-notifiable infectious diseases, monitored through hospitalisation and mortality data rather than routine notification, account for 92% of infectious disease hospitalisations.
Key Terms
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Infectious Disease
A disease caused by pathogenic microorganisms such as bacteria, viruses, parasites, or fungi.
Communicable Disease
A disease that can be communicated or transmitted from one person to another.
Epidemiologic Triad
A traditional model for infectious disease consisting of an external agent, a susceptible host, and an environment that brings them together.
Chain of Infection
The sequence by which an infectious agent leaves its reservoir, is transmitted, and enters a susceptible host to cause infection.
Incubation Period
The time between infection with a pathogen and the onset of symptoms.
Carrier
A person with inapparent infection who is capable of transmitting the pathogen to others without showing symptoms.
Herd Immunity
Community-level protection achieved when a sufficient proportion of the population is immune to an infectious disease.
Endemic
A disease that is constantly present at a predictable, stable level within a population or geographic area.
Epidemic
A sudden increase in the number of disease cases above what is normally expected in a particular population, place, or time period.
Contact Tracing
A public health process used to identify, assess, and manage people who have been exposed to a person with an infectious disease.
Sentinel Surveillance
A system using selected people, places, or data sources to monitor specific infectious diseases and detect early warning signs.
Vaccination
The administration of a vaccine to stimulate the immune system to recognise and fight specific pathogens without causing disease.
Matching Game
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End of Week Test
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Paramedicine Case Study
Apply this week's concepts to a realistic paramedicine scenario.
Lecture Materials
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