An epidemic curve shows a sharp single-peak rise in cases over one to two days, with all onsets falling within one incubation period of each other. This pattern is most consistent with which type of exposure?
APropagated person-to-person spread
BPoint-source exposure
CContinuous common-source exposure
DVector-borne transmission with long extrinsic incubation
A narrow, single-peak epidemic curve in which all cases fall within one incubation period indicates that everyone was exposed at essentially the same moment—a point-source event. Propagated spread produces successive waves of cases with each peak separated by roughly one incubation period. Continuous-source outbreaks show a plateau rather than a sharp peak.
Question 2 True / False
Control measures in an outbreak investigation should be delayed until analytic studies confirm the exposure source, to avoid implementing the wrong intervention.
TTrue
FFalse
Answer: False
Investigation and control proceed simultaneously—waiting for complete confirmation risks ongoing cases and deaths. Control measures can be targeted at the most plausible hypothesis based on descriptive data, then refined as analytic evidence accumulates. This is a core principle of field epidemiology: the cost of acting on a strong hypothesis is far lower than the cost of delay.
Question 3 Short Answer
Why is a sensitive case definition more appropriate at the beginning of an outbreak investigation, while a more specific one becomes important later?
Think about your answer, then reveal below.
Model answer: Early sensitivity ensures that all potential cases are captured so the full scope and pattern of the outbreak can be characterized. If the definition is too restrictive initially, cases are missed, the epidemic curve is distorted, and hypotheses about source may be wrong. Once the vehicle or exposure is identified, tightening specificity confirms which cases are truly linked to it, reducing false positives in attack-rate analyses.
Case definition is a tool that serves different analytical goals at different stages. Sensitivity-first prevents undercounting when the outbreak's source is unknown; specificity-later prevents overcounting when you are trying to prove a statistical association between exposure and illness.