Checkpoint Quiz 1, Causes of Infectious Disease

• 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).
• 16, A: CFU/mL = 62 ÷ (0.1 × 10⁻²) = 62 ÷ 0.001 = 62,000 CFU/mL.
• 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.