Types of Pathogens
You'll be able to classify a disease by the type of pathogen that causes it.
In 2023, CSIRO researchers identified 5 distinct pathogen types responsible for Australia's most common infections, bacteria, viruses, fungi, protists, and parasitic worms, each demanding a completely different treatment strategy.
Printable Worksheets
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Q1 · List three different topics you have learned about disease so far, and explain how they might connect to each other.
Q2 · What do you think makes a good scientific investigation question? How is it different from a question you could just Google?
● Know
- Identify the five main types of pathogen, bacteria, viruses, fungi, protists, and parasitic worms
- Recall an example disease caused by each type of pathogen
- Recognise the ways pathogens cause harm, including toxins, cell destruction, and immune evasion
● Understand
- Explain the structural differences between bacteria, viruses, fungi, protists, and worms
- Describe how toxins and other mechanisms damage host tissues
- Compare why bacterial and viral infections need different treatments
● Can do
- Classify a disease by the type of pathogen that causes it
- Match each pathogen type to a suitable treatment
- Explain why antibiotics do not work against viruses
Imagine a hospital ward in 2023: one patient has a bacterial lung infection treated with antibiotics, the patient beside them has a viral cold that antibiotics cannot touch, and a third has a fungal skin infection requiring a completely different drug. This is the practical reality that pathogens create, organisms that cause disease, and they come in five main types, each with distinct structures and behaviours that demand different treatments.
Bacteria
Single-celled prokaryotes with no nucleus. They reproduce rapidly by binary fission. Some bacteria are beneficial (gut flora), but pathogenic bacteria cause diseases like tuberculosis, strep throat, and urinary tract infections. Bacteria can be treated with antibiotics.
Viruses
Non-cellular particles consisting of genetic material (DNA or RNA) inside a protein coat. They cannot reproduce independently, they hijack host cells to replicate. Viral diseases include influenza, COVID-19, measles, and HIV. Antibiotics do NOT work on viruses.
Fungi
Eukaryotic organisms including yeasts and moulds. Fungal infections include athlete foot, ringworm, and thrush. They are treated with antifungal medications.
Protozoa
Single-celled eukaryotes. Malaria (Plasmodium) and giardiasis are protozoan diseases. They are treated with antiparasitic drugs.
Parasites (helminths)
Multicellular worms including tapeworms and hookworms. They are treated with antiparasitic medications.
Text description
- Bacteria: about 1–5 μm, with a cell wall; e.g. tuberculosis (TB); treated with antibiotics.
- Virus: about 0.1 μm, not a cell; e.g. flu; treated with antivirals.
- Fungus: made of hyphae, a eukaryote; e.g. athlete's foot; treated with antifungals.
- Protist (protozoan): has a nucleus, about 10–100 μm; e.g. malaria; treated with antiparasitics.
- Parasitic worms (helminths): multicellular eukaryotic parasites; e.g. tapeworm; treated with antiparasitic medicines.
- Size comparison shown: protist > bacteria >> virus.
Streptococcus pyogenes is a bacterium that causes strep throat. It is a spherical bacterium (coccus) that chains together and produces toxins that damage throat tissues. In contrast, SARS-CoV-2 (the virus that causes COVID-19) is a tiny particle about 100 times smaller than Streptococcus. It has RNA genetic material inside a lipid envelope studded with spike proteins. It cannot reproduce on its own, it must enter human cells and hijack their machinery. This fundamental difference explains why strep throat is treated with antibiotics (which target bacterial cell walls) while COVID-19 is treated with antivirals and supportive care, antibiotics would be completely ineffective against a virus.
Australian pathogen surveillance: The National Notifiable Diseases Surveillance System (NNDSS) tracks infectious diseases across Australia, categorising them by pathogen type. NSW Health and the Burnet Institute monitor emerging pathogens including drug-resistant bacteria and novel viruses. Australia unique wildlife harbours viruses with pandemic potential (like Hendra virus), and the Australian Centre for Disease Preparedness at CSIRO Geelong researches these pathogens to develop diagnostics and vaccines. Understanding pathogen classification is essential for Australia biosecurity and pandemic preparedness.
Match each pathogen type to an example disease it causes.
Pathogens cause disease through several mechanisms:
1. Toxin production: Many bacteria produce toxins, poisonous substances that damage host tissues. Exotoxins are proteins secreted by living bacteria. Examples: tetanus toxin (causes muscle spasms), cholera toxin (causes severe diarrhoea), botulinum toxin (causes paralysis, also used in Botox). Endotoxins are lipopolysaccharides in the outer membrane of Gram-negative bacteria, released when bacteria die. They trigger fever and inflammation.
2. Cell destruction: Some pathogens directly destroy host cells. Viruses replicate inside cells until they burst (lyse), releasing new virus particles. The common cold virus destroys nasal epithelial cells, causing runny nose and congestion.
3. Immune system hijacking: Some pathogens evade or manipulate the immune system. HIV infects and destroys T-helper cells, crippling the immune response. Some bacteria form biofilms, protective slime layers that shield them from immune attack and antibiotics.
Clostridioides difficile (C. diff) is a bacterium that causes severe diarrhoea, particularly in hospital patients who have received antibiotics. It produces two toxins (TcdA and TcdB) that damage the intestinal lining and trigger inflammation. Normally, harmless gut bacteria keep C. diff under control. But broad-spectrum antibiotics kill these protective bacteria, allowing C. diff to multiply and produce toxins. This is why C. diff infections are a major problem in healthcare settings. Australian hospitals monitor C. diff rates closely and use targeted antibiotic stewardship to prevent outbreaks. The NSW Clinical Excellence Commission publishes guidelines for C. diff prevention and management.
Australian toxin research: The Queensland Museum and University of Queensland researchers study Australian venomous animals (snakes, spiders, jellyfish, cone snails) whose toxins have yielded valuable medical compounds. Captopril (a blood pressure drug) was developed from jararaca snake venom. Prialt (a painkiller) comes from cone snail venom. While these are animal toxins rather than pathogen toxins, they illustrate how understanding toxin structure and function leads to medical breakthroughs. Australian researchers also study bacterial toxins to develop new antibiotics and antitoxin therapies.
One of the most important distinctions in medicine is between bacterial and viral infections, because the treatments are completely different.
Bacterial infections can be treated with antibiotics drugs that kill bacteria or stop them from reproducing. Antibiotics target structures unique to bacteria, such as cell walls (penicillins), ribosomes (macrolides), or DNA replication (fluoroquinolones). Because human cells do not have these bacterial structures, antibiotics can kill bacteria without harming human cells.
Viral infections cannot be treated with antibiotics because viruses lack the structures antibiotics target. Viruses use host cell machinery to replicate, so killing the virus without killing the host cell is difficult. Antiviral medications target specific viral enzymes (like neuraminidase in influenza or reverse transcriptase in HIV) but are less effective than antibiotics and must be given early.
Why the distinction matters: Taking antibiotics for viral infections (like colds and flu) is useless, causes side effects, and drives the evolution of antibiotic-resistant bacteria, one of the greatest threats to global health.
During the COVID-19 pandemic, some people demanded antibiotics from their doctors, believing they would help. They did not, COVID-19 is caused by a virus (SARS-CoV-2), and antibiotics have no effect on viruses. In fact, widespread antibiotic use during the pandemic may have accelerated antibiotic resistance. Australian data from the NPS MedicineWise program showed that antibiotic prescribing for respiratory infections actually decreased during the pandemic, partly because lockdowns reduced circulating infections and partly because public awareness improved. However, inappropriate antibiotic use remains a major problem globally, with the World Health Organisation estimating that 50% of antibiotics are prescribed unnecessarily.
Australian antibiotic stewardship: Australia has one of the highest rates of antibiotic prescribing in the OECD. The Australian Commission on Safety and Quality in Health Care runs the National Antimicrobial Resistance Strategy, promoting appropriate antibiotic use in hospitals and communities. The NPS MedicineWise program educates the public that antibiotics do not treat viral infections. Australian researchers at the Peter Doherty Institute study how antibiotic resistance spreads and develop rapid diagnostics to distinguish bacterial from viral infections, ensuring antibiotics are only used when truly needed. This work is critical for preserving the effectiveness of existing antibiotics.
Find the error in this student advice about treating illness.
- Colds are caused by viruses, not bacteria.
- Antibiotics only work against bacteria, not viruses.
- Taking antibiotics for a viral infection contributes to antibiotic resistance.
- Sharing prescription antibiotics with friends is safe and effective.
Wrong: "A depth study is just a long essay about a disease." No, a depth study is an investigation. It requires you to ask a question, gather evidence, analyse data, and draw conclusions. It is active science, not just research.
Right: A depth study is an active scientific investigation where you pose a question, gather and analyse evidence, and draw conclusions. It is hands-on inquiry, not just a literature review or essay.
Wrong: "The different topics in this unit have no connection to each other." No, they are deeply connected. Pathogens cause disease, which the immune system fights, which vaccines train, which antibiotics treat, which resistance limits, which public health prevents. Every topic links to others.
Right: The topics in this unit form an integrated network: pathogens cause disease, the immune system fights back, vaccines train immunity, antibiotics treat bacterial infections, resistance limits treatments, and public health strategies prevent spread. Each concept connects to the others.
Wrong: "Once you memorise facts about disease, you understand it." No, true understanding means being able to explain connections, apply concepts to new situations, and evaluate evidence. Facts are tools; understanding is the ability to use them.
Right: True understanding in science means you can explain connections between concepts, apply ideas to new situations, and evaluate evidence critically. Memorised facts are only useful when you know how to use them.
Australian Scientists Fighting Disease
Professor Fiona Stanley (AC): An Australian epidemiologist who founded the Telethon Kids Institute in Perth. Her research on birth defects, Indigenous health, and population health methods transformed Australian public health. She championed the use of population data to guide health policy.
Professor Ian Frazer: Co-developer of the HPV vaccine at the University of Queensland. His work has prevented countless cases of cervical cancer worldwide and put Australia on track to eliminate cervical cancer entirely.
Modern Australian research: Today, Australian scientists at WEHI, the Doherty Institute, CSIRO, and universities across the country continue to fight disease. During COVID-19, Australian researchers contributed to vaccine development, genomic surveillance, and long COVID research. Aboriginal and Torres Strait Islander researchers are increasingly leading health research that addresses community priorities with cultural authority.
✍ Copy Into Your Books
▾Unit Connections
- Pathogen -> Transmission -> Defence -> Treatment
- Infectious vs non-infectious disease
- Local, national, and global perspectives
Key Formulas
- Herd immunity threshold ≈ 1 - 1/R0
- Incidence rate = (new cases/population) × multiplier
- Case fatality rate = (deaths/cases) × 100%
Depth Study Steps
- Choose topic -> Formulate question -> Research -> Hypothesis -> Method -> Data collection -> Analysis -> Conclusions -> Communication
Classify Pathogens Using Evidence
Explain Harm and Evaluate Treatment Evidence
At the start of this lesson, you thought about how wildly different organisms, bacteria, viruses, fungi, and worms, all have the power to cause disease despite being as unrelated as a tree is to a smartphone.
Now that you've worked through the lesson, what do these different pathogens actually have in common that makes them capable of causing illness? Did anything surprise you?
Q1. Name the five main types of pathogen and give one example disease caused by each. (2 marks)
Q2. Explain two different ways that pathogens cause harm to the body, using an example for each. (3 marks)
Q3. A doctor tells a patient that antibiotics will not help their cold. Explain why this is correct, referring to the type of pathogen involved. (3 marks)
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- How has your understanding of disease and health developed across this entire unit?
- What connections between concepts do you find most powerful or surprising?
Model answers (click to reveal)
Answers
▾MCQ 1
A A virus is not made of cells. It is a particle of genetic material in a protein coat that can only reproduce by invading a host cell and hijacking its machinery.
MCQ 2
C Plasmodium is a single-celled organism (a protozoan) spread by mosquitoes. Malaria is the classic protozoan disease.
MCQ 3
B A toxin is a poisonous substance produced by some pathogens (often bacteria) that damages body cells and causes the symptoms of disease.
MCQ 4
D Antibiotics target structures unique to bacteria, such as cell walls and bacterial ribosomes. Viruses are not cells and do not have these structures, so antibiotics have no effect on them.
MCQ 5
A Tinea (athlete's foot) and thrush are fungal infections, caused by fungi such as yeasts and moulds, and are treated with antifungal medications.
Short Answer 1
Model answer: The five main types of pathogen are: Bacteria, single-celled organisms with no nucleus (e.g. tuberculosis or strep throat); Viruses, non-living particles that hijack host cells to reproduce (e.g. influenza or COVID-19); Fungi, including yeasts and moulds (e.g. tinea / athlete's foot or thrush); Protozoa, single-celled organisms often spread by water or insects (e.g. malaria); and Parasitic worms (helminths), multicellular worms (e.g. tapeworm or hookworm). Award marks for correctly naming the types and matching a sensible example disease to each.
Short Answer 2
Model answer: Pathogens harm the body in several ways. Toxin production: many bacteria release toxins, poisonous substances that damage host tissues, for example tetanus toxin causes painful muscle spasms and cholera toxin causes severe diarrhoea. Cell destruction: some pathogens directly destroy host cells, for example a cold virus replicates inside nasal cells until they burst (lyse), releasing new virus particles and causing a runny nose. Immune evasion: some pathogens damage or hide from the immune system, for example HIV infects and destroys T-helper cells, weakening the body's defences. A full-mark answer clearly explains two of these mechanisms and links each to a specific example.
Short Answer 3
Model answer: The doctor is correct because a cold is caused by a virus, not a bacterium. Antibiotics work by targeting structures that are unique to bacteria, such as bacterial cell walls and ribosomes. Viruses are not living cells and do not have these structures, instead they invade the body's own cells and use the host's machinery to reproduce. Because there is no bacterial cell wall or ribosome for the antibiotic to attack, antibiotics have no effect on the virus that causes a cold. Taking antibiotics for a viral infection is therefore useless, can cause side effects, and helps drive the development of antibiotic-resistant bacteria.
Activity 1: Classify Pathogens Using Evidence
- Accept: bacteria are single cells without a nucleus, e.g. tuberculosis or strep throat; viruses are non-cellular particles containing genetic material in a protein coat, e.g. influenza or COVID-19; fungi have cells with nuclei and include yeasts and moulds, e.g. tinea or thrush; protozoa are single-celled organisms with nuclei, e.g. malaria caused by Plasmodium; parasitic worms are multicellular animals, e.g. tapeworm or hookworm infection. A bacterium is a cell and can reproduce by cell division under suitable conditions; a virus depends on a host cell's machinery to replicate. Credit accurate alternatives from the lesson. Do not require students to describe all fungi as multicellular or all bacteria as harmful.
- A is a virus: it is non-cellular, has genetic material in a protein coat and depends on a host cell for replication. B is a bacterium within the five groups used here: it is a single cell without a nucleus. C is a protozoan: Plasmodium is the malaria-causing single-celled eukaryote taught in the lesson. Both B and C are single-celled, so cell number alone does not distinguish them; the nucleus does. A nucleus alone would not identify every unknown organism as a protozoan, because fungi also have nuclei; C's named example supplies the additional evidence.
- Plasmodium is the disease-causing pathogen, classified here as a protozoan. The infected mosquito carries and transmits the parasite during a bite; it is the carrier or vector rather than the malaria pathogen. Naming 'vector' is optional because the role is supplied in the question. Do not infer that all mosquitoes carry malaria or that the mosquito itself is a protozoan.
Activity 2: Explain Harm and Evaluate Treatment Evidence
- For example: cholera-causing bacteria produce a toxin that disrupts normal intestinal function, causing severe diarrhoea; or tetanus-causing bacteria produce a toxin that disrupts nerve control of muscles, causing spasms. HIV is a virus that infects and damages T-helper cells, weakening the body's immune defences. Credit the level of mechanism taught: students need not explain molecular signalling or detailed immune coordination. A pathogen is the infectious disease-causing agent. A toxin is a harmful substance produced by an organism; in these examples it is produced by bacteria. A toxin is not itself a bacterium or virus. Credit another accurate toxin example from the lesson.
- Antibiotics can treat certain bacterial infections; antivirals are used for some viral infections; antifungals treat fungal infections. The specified antibiotic targets bacterial cell-wall construction. Viruses are not bacterial cells and have no bacterial cell wall, so they lack that target. Different pathogens and medicines have different properties, and not every infection requires or responds to a medicine in its category. These are scientific categories, not advice to choose or take a particular drug. Accept the lesson's examples without requiring names of medicines, doses or clinical decisions.
- In these fictional systems, the antibiotic reduced bacterial growth relative to its matched control but did not reduce viral replication. This is consistent with a drug targeting bacterial cell-wall construction, a target viruses lack. Suitable controlled conditions include the duration of observation, temperature, starting amount of the relevant organism, or host-cell type/number within the viral comparison; compare like with like within each system, not bacterial cells directly with virus particles. The result concerns only the tested bacterium, virus, conditions and antibiotic. It cannot establish effectiveness against every bacterium, nor human safety, because other bacteria and effects in people were not tested. The 20% value describes remaining growth relative to a control; it does not prove that 80% of bacteria were killed. This is supplied model data for reasoning, not a practical method.