4. Epidemic Curves, Spot Maps, and Surveillance: Reading Data
After this lesson you can
- Describe a problem by person, place and time and pick the display that fits each question
- Build an epidemic curve by date of onset and classify its shape
- Estimate an exposure window from the earliest and latest onsets and the incubation range
- Compare passive, active, sentinel, syndromic and enhanced surveillance systems
- Write a tiered case definition and compute system sensitivity and positive predictive value
- Run an alert threshold and the artifact checklist before calling a rise an outbreak
A lesson on describing disease across a population and watching it over time. It covers person, place and time, choosing the right chart or map, building and reading epidemic curves, and back-calculating an exposure window from a fictional picnic outbreak. It then explains surveillance systems, tiered case definitions, and the artifact checklist used before calling a rise in reports an outbreak.
Questions and answers
Questions a learner would ask about this lesson. Think through your own answer before you open each one.
- Why should an epidemic curve be built by date of onset and not report date?
Show answer
Onset is the biological event that the incubation period ties to the exposure. Report, diagnosis and laboratory dates are administrative and arrive after onset by a variable delay. A curve by report date shows weekend gaps and backlog spikes that have nothing to do with the disease, so report dates are used only to measure reporting delay.
- How do I estimate the exposure window from an epidemic curve?
Show answer
Subtract the minimum incubation period from the earliest onset, and subtract the maximum incubation period from the latest onset. For the Oakhaven picnic, hour 25 minus 12 is hour 13, and hour 59 minus 48 is hour 11, so exposure was about hours 11 to 13. Because incubation ranges and recalled onset times are approximate, investigators then widen that window.
- Why can a spot map mislead?
Show answer
A spot map plots one dot per case, so dots pile up wherever people pile up. A choropleth map shades areas by rate, which shows where risk is. In the lesson, District North and District South both have 100 cases, but South has half the population, so its rate is 200 per 100,000 against North's 100 per 100,000.
- What is the difference between passive and active surveillance?
Show answer
The difference is who initiates the report. In passive surveillance the provider sends the report on their own; in active surveillance the agency calls or visits to ask for cases. Passive is cheaper and less complete, active is more complete and costlier, and passive does not mean the agency does nothing.
- What should I check before calling a jump in reports an outbreak?
Show answer
Run the artifact checklist: a changed case definition, a new test, a changed reporting rule or system, an awareness campaign, a changed population, or batch reporting and a cleared backlog. An alert from a threshold is a reason to check, not yet an outbreak. In Oakhaven every checklist answer was no, so the rise in week 10 was treated as real.
Practice quiz
- Question 1An epidemic curve should plot cases by:
- Question 2A curve with a steep rise, a slower fall, and all cases within one maximum incubation period suggests:
- Question 3The earliest onset is at hour 25 with a minimum incubation of 12 hours, and the latest onset is at hour 59 with a maximum incubation of 48 hours. The exposure window is about hours:
- Question 4A health department telephones clinics each week to ask for cases. This is:
- Question 5Same-week counts in five past years were 10, 12, 16, 17 and 20 (mean 15, standard deviation 4). With an alert threshold of the mean plus two standard deviations, which current count triggers an alert?
- Question 6A complete audit finds 200 true cases in Oakhaven in a year, and the surveillance system reported 50 of them. What is the system's sensitivity?
- Question 7Oakhaven's syndromic system flags 80 reports in a week, and 20 turn out to be true cases. What is the positive predictive value?
- Question 8A curve shows peaks spaced about one incubation period apart, and the peaks grow. Which pattern is this?
- Question 9In the nested case definition tiers, a case that meets clinical criteria plus an epidemiologic link, without laboratory confirmation, is:
- Question 10Broadening a surveillance case definition usually does what?
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Principles of Epidemiology is education in epidemiologic reasoning, not clinical, screening, or policy guidance. Every example is set in the fictional Oakhaven Health District, and every number is invented for teaching.