Pathogens and disease transmission - TCE Biology (Tasmania)
“Pathogens: biological pathogens include prions, viruses, bacteria, fungi, protists and parasites; infectious disease is caused by invasion of a pathogen, which can be transmitted from one host to another; adaptations of pathogens for entering cells and tissues and for transmission (direct contact, body fluids, contaminated food or water, vectors); the physical or chemical changes that stimulate immune responses; factors affecting the spread of disease”
An infectious disease is caused by a pathogen invading a host and can be passed from one host to another. The course names six types of pathogen: prions (misfolded proteins), viruses (genetic material in a protein coat that reproduces only inside host cells), bacteria (prokaryotic cells), fungi, protists (single-celled eukaryotes) and parasites (such as worms and lice). Pathogens spread by direct contact, contact with body fluids, contaminated food or water, the air, or vectors such as mosquitoes. They have adaptations for entering hosts and spreading, and their foreign molecules and toxins trigger the immune response.
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What this dot point is asking
Pathogens are part of criterion 7 (Section D of the TASC exam). The external assessment specifications include the types of pathogen, a general understanding of common infectious diseases, the modes of transmission and the life cycles of some parasites. How each disease actually causes illness is excluded.
The six types of pathogen
| Pathogen | What it is | Examples of disease |
|---|---|---|
| Prions | Misfolded proteins that cause normal proteins to misfold; not living | Creutzfeldt-Jakob disease; bovine spongiform encephalopathy (mad cow disease) |
| Viruses | Genetic material (DNA or RNA) in a protein coat; reproduce only inside host cells | Influenza, measles, COVID-19 |
| Bacteria | Single-celled prokaryotes; many release toxins | Tuberculosis, cholera, Salmonella food poisoning |
| Fungi | Eukaryotes that grow as threads or yeasts, often on skin or moist surfaces | Tinea (athlete's foot), thrush; chytrid fungus in frogs |
| Protists | Single-celled eukaryotes | Malaria (Plasmodium), giardiasis (Giardia) |
| Parasites | Organisms living in or on a host and harming it (the course lists them separately) | Tapeworms, roundworms, head lice, ticks |
How pathogens are transmitted
- Direct contact: touching an infected person or animal (tinea, head lice).
- Body fluids: blood, saliva or sexual fluids (HIV, hepatitis B).
- Contaminated food or water: eating or drinking the pathogen (cholera, Salmonella, Giardia).
- Air: droplets or aerosols from coughing and sneezing (influenza, measles, tuberculosis).
- Vectors: an organism, often an insect, that carries the pathogen between hosts (mosquitoes carrying malaria or dengue).
Pathogen adaptations
Pathogens have adaptations that help them enter cells or tissues and spread:
- surface proteins that bind to receptors on specific host cells (viruses);
- toxins that damage tissue or cause symptoms that spread the pathogen, such as diarrhoea (cholera) or coughing and sneezing (respiratory viruses);
- protective capsules or cell walls that resist host defences (some bacteria);
- cysts or spores that survive outside a host (Giardia, fungi);
- complex life cycles using a vector (Plasmodium).
Life cycle of Plasmodium (malaria). An infected female Anopheles mosquito injects the parasite when it bites. The parasite multiplies in the liver, then invades red blood cells, multiplies again and bursts them, causing cycles of fever. Some parasites form sexual stages that are taken up by another mosquito, reproduce inside it and move to its salivary glands, ready to infect the next person.
How pathogens trigger an immune response
When a pathogen enters a host, it causes physical or chemical changes: foreign molecules (antigens) on its surface are recognised as "non-self", and toxins or damaged cells release chemical signals. These changes stimulate the innate and adaptive immune responses.
The spread of disease in populations
The course asks you to consider the factors that encourage or reduce spread, and how outbreaks can be predicted, monitored and contained.
| Factor | Encourages spread | Reduces spread |
|---|---|---|
| Persistence of the pathogen in hosts | Long infectious period; carriers without symptoms | Rapid diagnosis and treatment |
| Transmission mechanism | Airborne or vector-borne spread | Masks, hygiene, clean water, vector control |
| Proportion of the population immune | Low vaccination rates | High vaccination rates (herd immunity) |
| Mobility of the population | Travel and crowding | Quarantine, isolation, travel restrictions |
The pathogen is the agent (the virus SARS-CoV-2); the disease is the illness it causes (COVID-19). Name both where you can.
A gastro outbreak on a school camp
Twenty students develop vomiting and diarrhoea two days after drinking from a creek.
- Likely transmission: contaminated water, possibly a protist such as Giardia or bacteria.
- Adaptation: cysts or bacteria survive in water outside a host; diarrhoea releases more of the pathogen into the environment, helping it spread.
- Control: boil or treat drinking water, improve handwashing and toilet hygiene, and isolate sick students to stop person-to-person spread.
Marker's note: name the transmission route, a pathogen adaptation and a specific control measure linked to that route.
- Calling viruses cells
- Viruses are not cells and cannot reproduce outside a host cell.
- Confusing the vector with the pathogen
- The mosquito is the vector; Plasmodium is the pathogen.
- Explaining the symptoms in detail
- How each disease causes illness is excluded; focus on the pathogen type, transmission and adaptations.
Exam-style questions
Questions in the style of TASC 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.
Original6 marksCompare viruses and bacteria as pathogens, referring to their structure, how they cause disease and one example of each.Show worked answer →
Two marks for each point of comparison.
- Structure
- Bacteria are single-celled prokaryotes with a cell wall, cytoplasm, ribosomes and circular DNA, and can reproduce on their own. Viruses are not cells: they are genetic material (DNA or RNA) inside a protein coat, and can only reproduce inside a host cell.
- How they cause disease
- Bacteria multiply in tissues and many release toxins that damage cells. Viruses enter host cells and use the cell's machinery to make new viruses, often destroying the cell as they are released.
- Examples
- Bacterium: tuberculosis, cholera or Salmonella food poisoning. Virus: influenza, measles or COVID-19.
Original5 marksDescribe the life cycle of Plasmodium, the protist that causes malaria, and explain one way the spread of malaria can be controlled.Show worked answer →
Life cycle (3 marks). An infected female Anopheles mosquito bites a person and injects Plasmodium in its saliva. The parasites travel to the liver, multiply, then enter red blood cells, where they multiply further and burst the cells, causing fever. Some develop into sexual forms that are taken up when another mosquito feeds; they reproduce in the mosquito and move to its salivary glands, ready to infect the next person.
Control (2 marks). Any one, explained: insecticide-treated bed nets stop mosquitoes biting at night; draining standing water removes breeding sites for the vector; antimalarial drugs reduce the parasite reservoir in people.