Pathogens and disease: WACE Year 12 Biology
“Classify pathogens and explain how infectious diseases are transmitted”
A pathogen is a disease-causing agent, and the main types are bacteria, viruses, fungi, protozoa and prions; infectious diseases are caused by pathogens that can be transmitted between hosts by direct contact, droplets, contaminated food or water, vectors or body fluids, whereas non-infectious diseases are not.
What this dot point is asking
SCSA wants you to classify pathogens, distinguish infectious from non-infectious disease, and explain the modes of transmission. A strong answer matches each pathogen type to examples and links transmission routes to how diseases spread through populations.
Infectious versus non-infectious disease
A disease is any condition that impairs normal functioning of an organism. Diseases fall into two broad groups:
- Infectious (communicable) diseases are caused by pathogens and can be passed from one organism to another (for example, influenza, tuberculosis, malaria).
- Non-infectious (non-communicable) diseases are not caused by transmissible pathogens and cannot be caught (for example, genetic disorders, cancers, nutritional deficiencies, cardiovascular disease).
This dot point focuses on infectious disease and its agents.
Types of pathogen
A pathogen is any agent that causes disease. The main groups are:
- Bacteria: single-celled prokaryotes. Some cause disease by releasing toxins or damaging tissue (for example, tuberculosis, tetanus, salmonella). Treatable with antibiotics.
- Viruses: non-cellular agents of nucleic acid in a protein coat. They are obligate intracellular parasites that hijack host cells to replicate, destroying them (for example, influenza, HIV, measles). Not affected by antibiotics.
- Fungi: eukaryotic organisms such as yeasts and moulds, causing diseases like tinea (athlete's foot) and thrush.
- Protozoa: single-celled eukaryotes, often spread by vectors (for example, Plasmodium, which causes malaria).
- Prions: misfolded proteins that cause other proteins to misfold, damaging nervous tissue (for example, the agent of mad cow disease). They contain no nucleic acid.
Larger parasites such as worms (helminths) can also cause disease.
Antibiotics target structures and processes specific to bacterial cells, such as cell walls and bacterial ribosomes. Viruses lack these structures and replicate inside host cells, so antibiotics have no effect on them. This is one reason antibiotics should not be prescribed for viral infections.
How pathogens cause harm
Pathogens cause disease (their pathogenicity) in several ways: by directly destroying host cells as they reproduce (many viruses), by releasing toxins that disrupt body functions (many bacteria), by competing for nutrients, or by triggering damaging immune responses. The severity often depends on the dose of pathogen and the host's resistance.
Modes of transmission
For an infectious disease to spread, the pathogen must move from an infected host to a new susceptible host. Common routes include:
- Direct contact: touching infected skin or sores, or sexual contact (for example, some skin infections, HIV).
- Droplet and airborne spread: inhaling droplets from coughs and sneezes (for example, influenza, tuberculosis).
- Contaminated food and water (faecal-oral): ingesting pathogens in food or water (for example, cholera, salmonella).
- Body fluids: blood, semen and other fluids, including via shared needles or transfusion (for example, HIV, hepatitis B).
- Vectors: an organism, often an insect, that carries the pathogen between hosts (for example, mosquitoes transmitting malaria).
- Fomites: contaminated surfaces or objects (door handles, utensils) that transfer pathogens.
Follow the transmission cycle
A female Anopheles mosquito bites an infected person and takes up Plasmodium in the blood meal. The parasite develops inside the mosquito (the vector). When the mosquito bites a healthy person, it injects the parasite into the bloodstream. The parasite infects liver cells and then red blood cells, causing the symptoms of malaria. Breaking any step (controlling mosquitoes, using nets, treating cases) interrupts the cycle.
Factors affecting spread
The spread of an infectious disease through a population depends on the mode of transmission, the density and movement of hosts, environmental conditions (warmth and moisture favour many pathogens and vectors), the level of immunity in the population, and the standard of hygiene, sanitation and public health measures.
Pathogen: a disease-causing agent. Host: an organism that a pathogen lives in or on. Vector: an organism that transmits a pathogen between hosts. Infectious disease: a disease that can be transmitted between organisms.
Why classifying pathogens matters
Knowing the type of pathogen determines how a disease is treated and controlled. Bacterial infections may respond to antibiotics; viral infections are managed with antivirals or prevented with vaccines; vector-borne diseases are controlled by targeting the vector. Identifying the mode of transmission tells public health authorities where to intervene, which is the link to epidemiology and disease control.
- Calling all microorganisms pathogens
- Most microbes are harmless or beneficial; only those that cause disease are pathogens.
- Saying antibiotics kill viruses
- Antibiotics work only against bacteria; they do nothing to viruses.
- Confusing a vector with the pathogen
- The mosquito is the vector; Plasmodium is the pathogen it carries.
- Treating genetic or lifestyle diseases as infectious
- Non-infectious diseases such as cancer or diabetes are not caused by transmissible pathogens and cannot be caught.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style6 marksClassify the five main types of pathogen, giving one example of a disease caused by each, and explain why antibiotics are effective against bacterial infections but not viral infections.Show worked answer →
A 6 mark answer needs the five types with examples plus the antibiotic reasoning.
- Types and examples
- Bacteria (e.g. tuberculosis or tetanus); viruses (e.g. influenza, HIV or measles); fungi (e.g. tinea or thrush); protozoa (e.g. Plasmodium causing malaria); prions (e.g. the agent of mad cow disease).
- Why antibiotics work on bacteria
- Antibiotics target structures and processes specific to bacterial cells, such as the bacterial cell wall and bacterial ribosomes, killing or stopping the bacteria.
- Why not viruses
- Viruses are non-cellular and lack these structures; they replicate inside host cells using the host's machinery, so antibiotics have nothing to act on and have no effect. This is why antibiotics should not be prescribed for viral infections.
Markers reward all five pathogen types with valid examples and the structural reason antibiotics affect bacteria but not viruses.
WACE 2024-style5 marksDescribe three different modes by which infectious diseases are transmitted between hosts, giving an example disease for each, and explain how identifying the transmission route helps control a disease.Show worked answer →
A 5 mark answer needs three routes with examples plus the control link.
Three routes (any three). Droplet or airborne spread, by inhaling droplets from coughs or sneezes (e.g. influenza or tuberculosis); contaminated food or water (faecal-oral), by ingesting the pathogen (e.g. cholera or salmonella); vector-borne, carried by an organism such as a mosquito (e.g. malaria); direct contact or body fluids (e.g. HIV).
Why it helps control. Knowing the actual route lets authorities target the right intervention: clean water and sanitation for waterborne disease, masks and distancing for droplet spread, and vector control (nets, insecticides) for vector-borne disease. Targeting the real transmission step makes control effective.
Markers reward three valid routes with examples and the point that matching the control measure to the transmission route makes it effective.