Covers Lessons 01–07: pathogen classification, Koch and Pasteur, modes of transmission, microbial testing, and agricultural disease in plants and animals.
Covers L01–L0720 MC · 3 Short Answer35 marks total
Lesson Summaries
L01 What Is Infectious Disease?
An infectious disease is caused by a pathogen and can be transmitted between hosts — distinguishing it from non-infectious diseases caused by genetic, lifestyle, or environmental factors. Pathogens are classified into three categories: microorganisms (bacteria, fungi, protozoa), macroorganisms (helminths, ectoparasites), and non-cellular pathogens (viruses, prions, viroids). Data on disease transmission is collected using primary methods (contact tracing, case reporting, serology) and secondary methods (historical records, published research).
Pathogen classification determines treatment strategy — antibiotics target bacteria, antivirals target viruses, antifungals target fungi, anthelmintics target helminths. Viroids are plant-only RNA pathogens with no protein coat; prions are protein-only animal pathogens with no nucleic acid. Pathogens have specific adaptations for host entry (fimbriae, spike proteins, keratinases, hooks) and transmission (respiratory triggering, long latency, vector dependence, resistant eggs).
viroidprionfimbriaeACSBL117ACSBL118
L03 Koch and Pasteur — Germ Theory
Pasteur's swan-neck flask experiment (1859) disproved spontaneous generation by showing microorganisms come from air, not broth. Koch's four postulates (1876–82) established the method for proving a specific organism causes a specific disease: (1) found in all diseased, (2) isolated in pure culture, (3) causes disease in healthy host, (4) re-isolated from new host. Limitations include: viruses cannot be cultured on artificial media, asymptomatic carriers violate postulate 1, ethical constraints on human infection.
swan-neck flaskKoch's postulatespure culturecausation vs correlation
L04 Modes of Transmission
Three modes of transmission: direct contact (skin-to-skin, respiratory droplets, sexual, blood-to-blood), indirect contact (fomites, contaminated water/food, airborne droplet nuclei), and vector transmission (biological vector: pathogen completes part of life cycle in vector, e.g. Plasmodium in Anopheles). Epidemic curve shapes: point source (single sharp peak), continuous source (sustained plateau), propagated (successive waves). John Snow's 1854 Soho cholera investigation established the core epidemiological methodology by mapping cases spatially without knowing the causative organism.
direct contactindirect contactbiological vectorepidemic curve
L05 Microbial Testing
Serial dilution and plate count method estimates bacterial concentration (CFU/mL). Steps: serial dilution (10⁻¹ to 10⁻⁴), inoculate agar plates, incubate inverted at set temperature, count colonies (30–300 range), calculate CFU/mL = colonies ÷ (volume × dilution factor). Negative control (sterile water) rules out equipment contamination — should give 0 colonies. Positive control confirms medium supports growth. E. coli is used as the indicator organism for faecal contamination in water because it is easy to culture and its presence signals other pathogens may be present.
Plant pathogens include fungi (wheat stem rust, myrtle rust — spread via airborne spores), bacteria (fire blight, crown gall — spread via insects/tools/water), viruses (TMV, banana bunchy top — no cure once infected, vector-controlled), and nematodes (root-knot nematode — soil-dwelling macroorganism). Economic effects: reduced yield and quality, increased production costs, loss of export markets, ecological disruption. Myrtle rust (Austropuccinia psidii) arrived 2010, infects 350+ Australian Myrtaceae species, and has listed two species as critically endangered.
Score updates as you answer questions. Aim for 16/20 (80%) before moving on.
Multiple Choice — 20 marks
One mark each. Select the best answer for each question.
Lesson 01 — What Is Infectious Disease?
1. Which of the following is an example of an infectious disease?
A Type 2 diabetes, caused by insulin resistance related to lifestyle and genetics
B Coronary artery disease, caused by plaque buildup in arteries over decades
C Tuberculosis, caused by Mycobacterium tuberculosis transmitted via respiratory droplets
D Melanoma, caused by UV-induced DNA mutations in skin cells
2. Prions are classified as non-cellular pathogens because:
A They are too small to be seen with a standard light microscope
B They consist only of misfolded protein — they have no nucleic acid, no cell membrane, and cannot metabolise independently
C They replicate by inserting their protein into the host cell's nucleus
D They infect only non-cellular structures such as bones and cartilage
3. A public health team interviews all people who attended a music festival and developed gastroenteritis, asking about their food choices, movements, and contacts during the event. This is an example of:
A Secondary data collection using published surveillance databases
B Secondary data collection using serology surveys
C Primary data collection using laboratory culture of pathogens
D Primary data collection using contact tracing and case investigation
4. A tapeworm (Taenia solium) lives in the human intestine, absorbing digested nutrients through its body wall. It is correctly classified as:
A A macroorganism — it is a multicellular helminth visible to the naked eye
B A microorganism — it is a protozoan that lives inside host cells
C A non-cellular pathogen — it cannot reproduce independently outside a host
D A microorganism — it is a fungus that absorbs nutrients through its hyphae
Lesson 02 — Classifying Pathogens
5. A student is told that a new plant disease is caused by a pathogen consisting only of a short circular RNA molecule with no protein coat. This pathogen is best classified as:
A A virus — RNA-based pathogens are all viruses
B A prion — protein-free pathogens are classified as prions
C A viroid — a non-cellular RNA pathogen with no protein coat that infects plants
D A bacterium — RNA is the primary genetic material of prokaryotes
6. The HIV virus has a long asymptomatic period — infected individuals remain infectious for years without knowing they carry the virus. Which transmission adaptation does this represent?
A An adaptation for host cell entry — HIV uses long incubation to allow deeper penetration into tissues
C An adaptation for transmission — the host remains infectious and active for years, maximising the number of new hosts exposed
B An adaptation for transmission — the host remains infectious and active for years, maximising the number of new hosts exposed
D An adaptation for immune evasion — the long latency prevents the host from recognising and destroying the virus
7. Why can the same antibiotic not be used to treat both a bacterial infection and a fungal infection?
A Antibiotics are too large to penetrate fungal cells, making them physically ineffective
B Fungi are resistant to all known antibiotics because they evolved before antibiotics were discovered
C Fungal infections are always located on body surfaces rather than internally, so oral antibiotics cannot reach them
D Antibiotics target bacterial cell structures (peptidoglycan cell walls, 70S ribosomes) that are absent in fungi — fungi have chitin cell walls and 80S ribosomes requiring different drug targets
8. Plasmodium falciparum develops inside the salivary glands of Anopheles mosquitoes before being injected into a human host during a blood meal. The mosquito in this scenario is acting as:
A A biological vector — Plasmodium completes part of its life cycle within the mosquito
B A mechanical vector — the mosquito carries Plasmodium on its body surface without hosting development
C A fomite — the mosquito is a contaminated object that passively delivers the pathogen
D An intermediate host only — the mosquito is the primary host and humans are secondary
Lesson 03 — Koch and Pasteur
9. In Pasteur's swan-neck flask experiment, the swan-neck flask remained clear (no microbial growth) while the straight-neck flask became turbid. The independent variable in this experiment was:
A The temperature at which the flasks were stored after sterilisation
B Whether airborne particles could reach the broth, controlled by the shape of the flask neck
C The composition of the nutrient broth placed in each flask
D The presence or absence of oxygen allowed into each flask
10. A researcher inoculates a healthy guinea pig with a pure culture of a suspected pathogen. The guinea pig develops the same disease as the original host. Which of Koch's postulates has been satisfied?
A Postulate 1 — the microorganism is found in all diseased organisms
B Postulate 2 — the microorganism is isolated and grown in pure culture
C Postulate 3 — the cultured microorganism causes disease in a healthy host
D Postulate 4 — the microorganism is re-isolated from the newly diseased host
11. Koch's postulates cannot be fully applied to prion diseases (such as CJD) because:
A Prions are too small to be isolated from diseased tissue
B Prions cause disease in humans only, making postulate 3 impossible to test ethically
C Prion diseases have such long incubation periods that testing Koch's postulates would take decades
D Prions consist only of protein with no nucleic acid — they cannot be grown in pure culture on any standard or artificial growth medium
12. Postulate 1 of Koch's postulates alone is insufficient to prove that a microorganism causes a disease because:
A An organism consistently present during disease may be a harmless commensal or secondary infection — its presence does not prove it is the cause
B The microorganism may only be found in diseased organisms and not in the environment, making it impossible to culture
C Postulate 1 applies only to bacterial diseases and cannot be used for viral or parasitic infections
D Finding an organism in diseased tissue requires laboratory equipment that was not available in Koch's era
Lesson 04 — Modes of Transmission
13. Measles spreads via droplet nuclei smaller than 5 µm that remain airborne for extended periods in enclosed spaces. This transmission mode is classified as:
A Direct contact — respiratory spread always involves direct contact between people
B Indirect contact — airborne droplet nuclei carry the pathogen through the air without direct host-to-host contact
C Vector transmission — air acts as a biological vector carrying the pathogen
D Direct contact — large respiratory droplets fall rapidly and infect nearby individuals
14. An epidemic curve shows a single sharp peak in cases occurring over two days, followed by rapid decline, with all cases within one incubation period of each other. This most likely indicates:
A A propagated outbreak caused by person-to-person spread from an initial case
B A continuous source outbreak where contamination persisted throughout the period
C A point source outbreak where all cases shared a single common exposure at one point in time
D A vector-borne outbreak where a mosquito population peaked during those two days
15. John Snow's 1854 cholera investigation was significant because it:
A First identified Vibrio cholerae as the causative organism of cholera using Koch's postulates
B Proved that miasma (bad air) from sewage was the cause of cholera in the Soho district
C Demonstrated that boiling water was sufficient to sterilise it and prevent cholera
D Identified the waterborne transmission route through spatial case mapping, before the causative pathogen was known
16. A researcher plates 0.1 mL of a 10⁻² dilution of a food sample onto a nutrient agar plate. After incubation, 62 colonies are counted. What is the estimated CFU/mL in the original sample?
A 62,000 CFU/mL
B 620 CFU/mL
C 6,200 CFU/mL
D 620,000 CFU/mL
Lesson 05 — Microbial Testing
17. A negative control plate in a water microbial testing investigation shows 8 colonies after incubation. The most appropriate conclusion is:
A The result is acceptable — negative controls may show up to 10 colonies and remain valid
B The sterile water used was not completely sterile and contained a small number of bacteria
C The investigation is invalid — the medium, equipment, or environment was contaminated, making results unreliable
D The positive control must also show unexpected results before the investigation is considered invalid
18. Why are agar plates incubated in an inverted position during microbial testing?
A To create anaerobic conditions that better support bacterial growth
B To prevent condensation from dripping onto colonies and causing them to spread, merge, or become uncountable
C To allow bacteria to settle evenly across the agar surface under gravity
D To prevent airborne contamination from entering the plate through the lid
Lessons 06–07 — Agricultural Disease
19. Australian Myrtaceae species are particularly vulnerable to myrtle rust (Austropuccinia psidii) compared to South American Myrtaceae because:
A Australian soils lack the nutrients needed for Myrtaceae plants to produce antifungal compounds
B The Australian climate is warmer and more humid, providing better conditions for fungal spore germination
C Australian Myrtaceae grow more densely, allowing spores to spread more easily between plants
D Australian Myrtaceae have had no evolutionary exposure to this pathogen and therefore lack any co-evolved resistance mechanisms
20. Which of the following is an indirect economic effect of a foot-and-mouth disease detection in Australia?
A Blisters on the feet and mouth of infected cattle reducing their feed intake and weight gain
B Suspension of Australian beef exports to Japan, South Korea, and other major markets
C Veterinary costs to diagnose and confirm the disease in infected animals
D Loss of infected animals that must be culled to prevent further spread
Short Answer — 15 marks
Answer all three questions. Write in full sentences.
1. A scientist investigating a new fish disease finds the same bacterium in all diseased fish. She grows it in pure culture and inoculates healthy fish — 80% develop the disease. She re-isolates an identical bacterium from the sick fish. Evaluate whether Koch's postulates have been fully satisfied in this investigation. (4 marks)
1 mark: postulates 1, 2, and 4 are satisfied (with reasoning) | 1 mark: postulate 3 is not fully satisfied (80%, not all healthy fish) | 1 mark: explanation of what postulate 3 requires | 1 mark: evaluative conclusion about the strength of evidence
2. Compare the transmission of tuberculosis (Mycobacterium tuberculosis) and malaria (Plasmodium falciparum). In your answer, classify the transmission mode of each, explain how each reaches a new host, and describe one control measure specific to each transmission route. (5 marks)
1 mark: TB — indirect airborne (droplet nuclei) with mechanism | 1 mark: malaria — vector transmission (biological vector — Anopheles) with mechanism | 1 mark: TB control measure linked to airborne route | 1 mark: malaria control measure linked to vector | 1 mark: explicit comparison identifying a key difference
3. Assess the causes and effects of one named plant disease and one named animal disease on agricultural production in Australia. For each disease, identify the pathogen type, describe how it causes damage, and evaluate the economic consequences for Australian producers. (6 marks)
3 marks for plant disease: pathogen type + mechanism of damage + economic consequences | 3 marks for animal disease: pathogen type + mechanism of damage + economic consequences
Model Answers
1 — C: Tuberculosis is infectious — caused by a pathogen (M. tuberculosis) and transmissible. Diabetes (A), coronary artery disease (B), and melanoma (D) are all non-infectious; they lack a pathogen and cannot be transmitted between individuals.
2 — B: Prions are misfolded proteins — they have no nucleic acid, no cell membrane, cannot metabolise, and cannot reproduce independently. They are non-cellular by definition. Their size (A) does not define classification. They do not replicate by inserting into nuclei (C). They cause neurological disease in brain tissue, not bones (D).
3 — D: Directly interviewing cases and tracing their contacts is contact tracing — a primary data collection method. The researchers are collecting the data themselves. Secondary data uses information collected by someone else.
4 — A: A tapeworm is a multicellular helminth — a macroorganism. It is visible to the naked eye, not a single-celled protozoan (B), not non-cellular (C), and not a fungus (D).
5 — C: A short circular RNA with no protein coat is a viroid — plant-only pathogens. Viruses have protein coats. Prions are protein, not RNA. Bacteria have DNA genomes in cells.
6 — B/C: Long asymptomatic period = transmission adaptation — the host remains active, sexually and socially, without knowing they are infectious, maximising exposure of new hosts over years. This is a transmission adaptation, not an entry or immune evasion mechanism. (Note: options B and C were accidentally duplicated in the question — both refer to the correct answer.)
7 — D: Antibiotics target bacterial-specific structures absent in fungi — primarily peptidoglycan cell walls and 70S ribosomes. Fungi have chitin cell walls and 80S ribosomes. Size (A) is irrelevant. Evolution of resistance (B) is incorrect — antifungals exist. Location (C) is incorrect — systemic fungal infections exist and are treated with oral/IV antifungals.
8 — A: Plasmodium completes its sexual reproduction phase inside the Anopheles mosquito salivary glands — making the mosquito a biological vector. A mechanical vector carries pathogen on its surface without hosting development. A fomite is non-living.
9 — B: The independent variable is what was deliberately changed: the shape of the flask neck, which controlled whether airborne particles could reach the broth. Temperature (A), broth composition (C), and oxygen (D) were kept constant.
10 — C: Inoculating a healthy host with pure culture and producing the disease is postulate 3. Finding in all diseased = postulate 1. Isolation and culture = postulate 2. Re-isolation from new host = postulate 4.
11 — D: Prions consist only of protein — no nucleic acid. They cannot be grown in pure culture on any growth medium because "growing" a pathogen on artificial media requires the pathogen to replicate, which requires nucleic acid. Prions propagate by inducing misfolding in host proteins — not by replicating as cells or viruses do.
12 — A: Association (postulate 1) does not equal causation. An organism found in all diseased hosts might be a commensal, a secondary opportunistic infection, or a coincidental finding. Postulate 3 — inoculating a healthy host — is what establishes causation. The other options are incorrect or irrelevant to the logical argument.
13 — B: Droplet nuclei <5 µm remain airborne — this is indirect transmission via air. Large droplets (>5 µm) are direct contact. Air is not a biological vector. Option D describes large droplets incorrectly associated with measles.
14 — C: Single sharp peak within one incubation period = point source. Propagated = successive waves. Continuous = sustained plateau. Vector-borne does not typically produce a two-day peak.
15 — D: Snow's significance was identifying waterborne transmission via spatial mapping before Vibrio cholerae was known (discovered 1883 by Koch). He did not apply Koch's postulates (A), did not support miasma theory (B), and did not test boiling (C).
17 — C: A negative control should show zero colonies — any growth indicates contamination of the medium or equipment, invalidating the investigation. There is no acceptable threshold for negative control growth (A is wrong). The sterile water explanation (B) misidentifies the source of contamination. The positive control result (D) is irrelevant to what a positive negative-control result means.
18 — B: Inverted plates keep condensation on the lid away from the colony surface, preventing spreading and merging of colonies. This does not create anaerobic conditions (A), affect bacterial settling (C), or significantly affect airborne contamination (D).
19 — D: The vulnerability is due to absence of co-evolutionary history — no natural selection for resistance has occurred in Australian Myrtaceae populations. Soil nutrients (A), climate (B), and plant density (C) are not the primary explanation.
20 — B: Export suspension is an indirect effect — it results from trading partners' response to the disease detection, not from the disease directly damaging animals or production. Blisters (A), veterinary costs (C), and culling losses (D) are all direct effects.
SA1: Postulates 1, 2, and 4 are satisfied. The same bacterium was found in all diseased fish (postulate 1), it was grown in pure culture (postulate 2), and an identical bacterium was re-isolated from the newly diseased fish (postulate 4). Postulate 3 is not fully satisfied: postulate 3 requires that the pure culture causes the disease when introduced into healthy hosts — the implication is that all, or essentially all, inoculated hosts develop the disease. In this investigation, only 80% of inoculated fish developed disease. This may reflect individual variation in fish immunity, differences in inoculation dose, or environmental factors — but it means the postulate is not completely met as stated. The overall evidence is strong but not conclusive: three of four postulates are satisfied, and an 80% causation rate provides substantial support for the bacterium as the causative agent. To strengthen the investigation, the researcher should repeat the inoculation with standardised doses and controlled conditions to determine whether 100% causation can be achieved.
SA2: Tuberculosis (M. tuberculosis) is transmitted via indirect airborne transmission: infected individuals release droplet nuclei smaller than 5 µm when coughing, sneezing, or speaking; these particles remain suspended in air for extended periods and can be inhaled by susceptible individuals in the same room or enclosed space. The control measure specific to this route is ventilation and negative-pressure isolation rooms — increasing air exchange reduces the concentration of airborne droplet nuclei, lowering the probability of inhalation. Malaria (Plasmodium falciparum) is transmitted by vector transmission: the Anopheles mosquito is a biological vector in which Plasmodium completes its sexual reproduction stage. During a blood meal, sporozoites from the mosquito's salivary glands are injected into the human bloodstream. The control measure specific to this transmission route is vector control — including insecticide-treated bed nets, indoor residual spraying of insecticides, and drainage of mosquito breeding sites — which interrupts transmission by preventing the vector from biting the human host. The key difference is that TB transmission involves inhalation of airborne particles released directly from a human source, while malaria requires the intervention of a living vector that cannot be bypassed — meaning isolation of human cases has no effect on malaria transmission.
SA3 (example using myrtle rust and FMD): Myrtle rust is caused by the fungus Austropuccinia psidii — a microorganism. It causes damage by releasing wind-dispersed urediniospores that land on young, actively growing Myrtaceae tissue; hyphae penetrate the leaf surface and destroy cells, forming orange-yellow pustules that prevent photosynthesis, stunt growth, and kill young tissue. Economic consequences for Australian producers include direct losses to the nursery industry (valued at ~$900 million annually, with many lines being Myrtaceae), losses to the bush food industry using native Myrtaceae fruits, and the listing of two species as critically endangered, threatening long-term production viability. No broad-scale cure exists, meaning management costs are ongoing. Foot-and-mouth disease is caused by Aphthovirus — a non-cellular pathogen (virus). It causes damage by replicating in epithelial cells of the mouth and feet of cloven-hoofed animals (cattle, sheep, pigs), causing painful blisters that prevent eating and walking, leading to rapid weight loss, reduced milk production, and — particularly in young animals — cardiac damage and death. Economic consequences for Australian producers are dominated by indirect effects: a single FMD detection would trigger immediate export bans in most major markets, costing an estimated $50–80 billion in lost export revenue. The 2001 UK outbreak demonstrated total costs exceeding £8 billion despite FMD's relatively low mortality. Australia's disproportionately export-dependent livestock sector makes FMD's market access consequences uniquely severe — far exceeding the direct cost of infected or culled animals.