Classify transmission evidence without assuming every STI spreads identically
Route analysis uses the named pathogen, relevant exposure and supported entry route. Separate route cards represent alternatives, not a sequence; exposure alone does not confirm infection.
Section B15.5 · Sexually transmitted infections
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall pathogens, transmission routes, immune defence, viruses, blood and the difference between prevention and treatment.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which information is needed for a supported route explanation?
What you will learn
- Match a stated pathogen with a supported route
- Avoid extending one pathogen model to every infection
Original teaching model. Cards show stated mechanisms and conditions.
| Feature | Meaning |
|---|---|
| First | Name the pathogen |
| Second | Identify relevant exposure |
| Third | Identify supported entry route |
| Evidence limit | Exposure is not confirmed infection |
Match a stated pathogen with a supported route
A route classification should start with the named pathogen and the stated exposure. Sexual contact can transmit STI pathogens, but different infections may involve infected fluids, mucosal contact or skin contact in different ways. A general STI definition does not supply every transmission mechanism for every organism. Use the evidence relevant to the infection named in the question.
For HIV, transmission requires certain fluids containing the virus reaching a relevant entry route, such as mucosal tissue, damaged tissue or direct entry to blood. Sexual transmission is one route; contaminated injection equipment and perinatal routes are other examples. These mechanisms are different from ordinary sharing of classroom space or touching intact skin.
Avoid extending one pathogen model to every infection
An exercise may stipulate a fluid exposure and a possible entry route. Such a model supports identifying a potential transmission pathway, not concluding that infection is inevitable after every exposure. Virus availability, prevention and treatment conditions matter. Conversely, inventing an unsupported route from a vague statement such as contact happened produces an inaccurate explanation.
The original classification key uses separate route cards rather than arrows claiming one route turns into another. A shared label such as STI does not mean all pathogens respond identically to a prevention method. In each answer name the pathogen, specify the route and keep a possible exposure distinct from a confirmed infection.
Sources for this lesson
Pause and explain
Should an HIV route model be applied automatically to every STI?
Worked example
Why is “contact occurred” insufficient to classify HIV transmission?
Show the worked solution
- Specify a relevant fluid and entry route.
- Separate contact from confirmed infection.
Answer A supported HIV pathway needs the relevant fluid and entry route; ordinary contact alone does not establish such a pathway or confirmed infection.
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.
State three categories needed to analyse a fictional transmission record.
Give me a hint
Use pathogen, exposure and entry.
Compare my reasoning
- Identify the named pathogen.
- Identify relevant exposure and possible entry tissue.
Named pathogen; relevant exposure or fluid; supported entry route.
Look for these in your work
- Give all three categories
- Do not use appearance as evidence
Explain why possible exposure is not the same as confirmed infection.
Give me a hint
Separate a route model from evidence of established pathogen presence.
Compare my reasoning
- Describe what the exposure model supports.
- Require suitable infection evidence for confirmation.
Exposure identifies a possible route under stated conditions; it does not by itself confirm that the pathogen established an infection.
Look for these in your work
- Distinguish possibility and confirmation
- Retain the stated conditions
A learner claims every STI has exactly the same skin and fluid routes as HIV. Evaluate the generalisation.
Give me a hint
A category can include different pathogens.
Compare my reasoning
- Recognise HIV as one pathogen.
- Require pathogen-specific mechanisms for other infections.
The claim is unsupported: different STI pathogens have different mechanisms, so an HIV route model cannot be automatically extended to every STI.
Look for these in your work
- Reject the overgeneralisation
- Name the missing pathogen-specific evidence
Common mistakes
- Use pathogen-specific routes.
- Potential exposure does not mean inevitable infection.
What can you explain now?
Should an HIV route model be applied automatically to every STI?
Compare with the explanation
No, pathogen mechanisms differ
The general category does not make all routes identical.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain match a stated pathogen with a supported route and avoid extending one pathogen model to every infection. Use the stated route, conditions and evidence before opening the solutions.
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.