Trace transmission through food water and animals
Explain indirect routes using contaminated material or an animal carrier.
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall transmissible disease.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which item is an intermediate vehicle?
What you will learn
- A host can acquire pathogens from material
- Animals can be part of a route
A host can acquire pathogens from material
Contaminated food and water can carry a pathogen into a new host when consumed. In a supplied faecal-contamination model, the pathogen leaves an infected host, enters water or food, and is then swallowed by another host. Water that appears clear is not necessarily free of pathogens; contamination concerns the agent rather than visible dirt alone.
Identify the host, the contaminated material and consumption as separate links. Food is not the pathogen and every illness after eating is not automatically infectious: a non-infectious toxin could also be a cause. Use the evidence that a disease-causing organism is present in the particular problem.
Animals can be part of a route
Animals can transmit pathogens between hosts. In a supplied vector model an animal acquires a pathogen from one host and later passes it to another, for example during feeding. The animal carrier is different from the pathogen it transfers; not every animal is a vector for every disease.
A fly that carries contaminated material onto food provides a different intermediate route from a biting vector. Read the event rather than assuming a bite whenever an animal appears. Multiple routes can occur in one scenario, so explain the actual chain and choose a control that interrupts a stated link.
Sources for this lesson
Pause and explain
Which statement preserves the carrier-agent distinction?
Worked example
An animal carries P from A to B during feeding. Is the animal itself necessarily P?
Show the worked solution
- Separate carrier from causal agent.
- P is the pathogen in the description.
Answer No; the animal carries pathogen P between hosts.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Trace a pathogen from infected faeces to another host through drinking water.
Give me a hint
Keep all three route stages.
Compare my reasoning
- Faeces contaminate water.
- Another host swallows pathogen-containing water.
Infected faeces → contaminated water → consumption by another host.
Look for these in your work
- Include contamination
- Include ingestion
A fly lands on contaminated waste then on uncovered food. Explain the animal and food roles.
Give me a hint
Two intermediate links are present.
Compare my reasoning
- The fly carries contaminated material.
- The food becomes a vehicle when eaten.
The fly transfers pathogen-containing material to food; eating that contaminated food can expose another host.
Look for these in your work
- Distinguish both links
- Avoid inventing a bite
A picture shows a mosquito but supplies no pathogen or transfer evidence. Can a particular disease be identified?
Give me a hint
An animal image is not proof of agent identity.
Compare my reasoning
- The carrier category does not establish a particular pathogen.
- A supplied agent and route would be needed.
No; the picture alone does not establish which pathogen or disease, if any, is involved.
Look for these in your work
- State the limit
- Name missing biological evidence
Common mistakes
- Equating clear water with safe water.
- Confusing a vector with the pathogen.
What can you explain now?
Which statement preserves the carrier-agent distinction?
Compare with the explanation
An animal can transfer a pathogen
They have different roles.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain a food route and an animal route without calling either the pathogen.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.