Respect time order before inferring a transmission path

Define the host evidence

Contact information can constrain possible transmission routes, but contact is not proof that a transmission occurred. Host susceptibility, barriers, timing, source viability and the mode of spread all matter. A shared environmental source or missing contact can generate patterns that resemble direct spread between the recorded nodes.

Connect mechanism and inference

The original anonymous network has contacts A→B at time 1, C→D at time 2 and B→C at time 3. A disputed A→C contact at time 1.5 can be included as a separate record scenario. The initial possible source is present at time zero. A directed edge is usable only after its source node has become reachable; the model assumes no waiting period or loss only to isolate temporal ordering.

Without the disputed edge, A can reach B at 1 and C at 3, but cannot use C→D at 2 because that contact occurred too early. An unordered graph would wrongly report a complete A–B–C–D path. Including A→C at 1.5 makes C reachable before its time 2 contact and therefore makes D temporally reachable.

This is a set of possible paths, not a reconstructed actual outbreak or a probability of infection. Independent evidence of timing, exposure, host response and shared-source alternatives is still required. No contact tracing of real people, pathogen exposure or clinical inference is requested; the exercise connects mechanism constraints with the limits of observational evidence.

Reconstruct the original record

With A as the initial source and no disputed contact, reachable nodes are A at 0, B at 1, C at 3: three including the source. D is not reachable under the chronological rules. Adding the earlier A→C edge makes all four reachable, without proving any actual transmission.

Predict and investigate

Predict first: Why can an apparent path fail once its contact times are restored?

Replay the contacts in chronological order and retain the earliest possible arrival at each node. Compare the disputed-edge scenarios, then explain what source or host evidence would be needed beyond reachability.

Defend a changed case

Apply the mechanism to a changed boundary

Choose C as the initial source at time zero without the disputed edge. Which nodes become reachable?

Retain the first response, units, uncertainty and any prior feedback exposure in the notebook before comparing this public explanation.

Compare a worked response

C and D are reachable, with D at time 2. The directed record gives no path back to A or B. Two reachable nodes indicate only a permissible route under the model, not two actual infections.

Retain bounded evidence

Preserve original assumptions, first responses, source identity and feedback exposure in the Microbiology evidence workspace. These are fictional nonclinical records, not personal medical advice, actual patient data, diagnosis, treatment or an invitation to collect biological samples. Actual practical and independent learner competence require separate evidence.

Primary mechanism context, checked 31 October 2026. Original teaching and numerical records; no source clinical cases or procedures copied.