Epidemic curves are a basic investigative tool because they are so informative (see Lesson 4).
• The epi curve shows the magnitude of the epidemic over time as a simple, easily understood visual. It permits the investigator to distinguish epidemic from endemic disease. Potentially correlated events can be noted on the graph.
• The shape of the epidemic curve may provide clues about the pattern of spread in the population, e.g., point versus intermittent source versus propagated.
• The curve shows where you are in the course of the epidemic — still on the upswing, on the down slope, or after the epidemic has ended. This information forms the basis for predicting whether more or fewer cases will occur in the near future.
• The curve can be used for evaluation, answering questions like: How long did it take for the health department to identify a problem? Are intervention measures working?
• Outliers — cases that don’t fit into the body of the curve —may provide important clues.
• If the disease and its incubation period are known, the epi curve can be used to deduce a probable time of exposure and help develop a questionnaire focused on that time period.
Drawing an epidemic curve. To draw an epidemic curve, you first must know the time of onset of illness for each case. For some diseases, date of onset is sufficient. For other diseases, particularly those with a relatively short incubation period, hour of onset may be more suitable (see Lesson 4).
Occasionally, you may be asked to draw an epidemic curve when you don’t know either the disease or its incubation time. In that situation, it may be useful to draw several epidemic curves with different units on the x-axis to find one that best portrays the data. For example, the epidemic curves shown in Figures 6.2b and 6.2c display the same data as in Figure 6.2a; the x-axis is measured in units of 12 hours in Figure 6.2a, 6 hours in Figure 6.2b, and 24 hours (1 day) in 6.2c. Figure 6.2d shows the same data one more time, but with stacks of squares that each represent one case.