Epidemic curve
- Concept
The epidemic curve is a histogram of cases by date of symptom onset; its shape tells how the outbreak spread.
What it is
The epidemic curve plots the number of cases (vertical axis) by date or time of symptom onset (horizontal axis). Its shape suggests the type of outbreak: point source (one peak, all cases within one incubation period), continuous or intermittent common source (a plateau or irregular peaks) and propagated (successive waves one incubation period apart). If the agent is known, it lets us estimate when exposure happened.
Worked example
- Data (made up): 50 people fall ill after a lunch on 3 March at 13:00; cases are counted in 6-hour intervals.
- 4 March: 06–12 h, 3 cases; 12–18 h, 12; 18–24 h, 20. 5 March: 00–06 h, 9; 06–12 h, 4; 12–18 h, 2.
- Check: 3 + 12 + 20 + 9 + 4 + 2 = 50 cases.
- Peak: 18–24 h on 4 March; cumulative cases are 3, 15 and 35, so the median (25th) case falls in that interval.
- Median incubation: from 13:00 on the 3rd to 18–24 h on the 4th is 29 to 35 hours; ≈ 32 hours.
- All cases appear between 17 and 53 hours after the lunch: a single peak within one incubation period.
- Shape: steep rise and slower fall, no later waves: a point-source common-source outbreak.
- Reading: the lunch is the likely exposure; a second wave about 32 hours later would suggest person-to-person spread.
Curve shape and type of outbreak
| Type | Exposure | Curve shape |
|---|---|---|
| Point source | Single and brief | One peak, within one incubation period |
| Continuous common source | Prolonged | Rise and a broad plateau |
| Intermittent common source | Repeated on and off | Irregular peaks |
| Propagated | Person to person | Successive, growing waves |
Markers
- Point source
- Continuous source
- Propagated
- Incubation
In practice
- The curve is updated daily during the outbreak: if cases keep appearing after the source is removed, transmission continues another way.
- The same curve helps explain the outbreak to the community and judge whether control measures worked.
Common pitfalls
- Plotting by date of report instead of date of onset: it delays and distorts the curve.
- Choosing intervals that are too wide or too narrow: the shape disappears; they are fitted to the agent's incubation period.
- Reading a flattening curve as the end of the outbreak while cases are still unreported.
Test yourself
Think of the answer, then tap the question to see it.
What does the curve of a point-source outbreak look like?
A single peak with a steep rise and a slower fall, with all cases within one incubation period.
What do successive waves one incubation period apart suggest?
A propagated outbreak, with person-to-person transmission.
How is the time of exposure estimated in a point-source outbreak?
By subtracting the agent's median incubation period from the date of the peak or of the median case.
Which date goes on the horizontal axis of an epidemic curve?
Each case's date (or time) of symptom onset, not the date of report.
Connections
Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.
Sources: Principles of Epidemiology | Lesson 1 - Section 11 (Centers for Disease Control and Prevention, 2012); Principles of Epidemiology: Lesson 6, Section 2 (Centers for Disease Control and Prevention, 2012).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.