Free tool · CIC® exam topic
Epi curve generator
Paste onset dates, one case per line, and get an epidemic curve you can download. Your data never leaves your browser.
38 cases · peak on Oct 5 (12 cases)
How to read an epidemic curve
CDC describes four classic shapes:
- Point source: a steep upslope and a more gradual downslope — everyone was exposed at about the same time (the example above).
- Continuous common source: exposure is prolonged, so the curve has a plateau instead of a peak.
- Intermittent common source: an irregular, jagged curve.
- Propagated (person-to-person): a series of progressively taller peaks one incubation period apart.
Tips
- Plot cases by date (or hour) of symptom onset, not by date of report or test.
- Pick an interval shorter than the incubation period; too wide an interval hides the shape.
- Keep an eye on the tail: late cases can be secondary cases or a continuing source.
- Annotate the exported chart with interventions (for example, the day enhanced cleaning started) when you present it.
Test yourself: a CIC-style question
What is the difference between propagated (person-to-person) and point-source outbreaks on an epidemic curve?
- Propagated outbreaks show a single sharp peak; point-source outbreaks show several peaks
- Point-source outbreaks show one sharp peak; propagated outbreaks show successive peaks about one incubation period apart
- Both patterns look the same on an epidemic curve
- Point-source outbreaks always produce more cases
Show answer
B. Point-source outbreaks show one sharp peak; propagated outbreaks show successive peaks about one incubation period apart
A point-source outbreak has a steep upslope and a more gradual downslope within about one incubation period. A propagated outbreak shows a series of progressively taller peaks one incubation period apart.
Questions like this come up in the Surveillance and Epidemiologic Investigation domain of the CIC® exam. The 654 questions in the app each come with a written explanation.
FAQ
- What is an epidemic curve?
- A histogram of the number of cases (y-axis) by time of symptom onset (x-axis). It shows the outbreak’s size and time course and suggests how it spread.
- Should I use days or weeks?
- Choose an interval shorter than the incubation period of the illness. Days suit most healthcare outbreaks; weeks suit illnesses with long incubation periods or outbreaks lasting months. For very short incubation periods, CDC notes that the hour of onset may be more suitable.
- Is my data uploaded anywhere?
- No. The chart is drawn in your browser and nothing you paste is sent to a server. Use onset dates only — no names or other identifiers.
Sources
Checked against these sources on October 21, 2026. For education and CIC® exam preparation only — not a substitute for your facility’s policies, NHSN’s official reports, or clinical judgment. Calculations run in your browser; nothing you enter is sent anywhere.