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Epidemic Curves   Return to Public Health theme page

1. Core Knowledge:

An epidemic curve gives a graphical display of the numbers of incident cases in an outbreak or epidemic, plotted over time. The form of the resulting distribution of cases can be used to propose hypotheses on the nature of the disease and its mode of transmission. The course of a population outbreak is most evident in infectious disease, but also occurs in situations such as a chemical spill leading to cases of respiratory disease.

Here are the major classes of epidemic curve.

A. Common Source Outbreaks.
Here the cases of disease arise from a single, shared or 'common' source, such as a batch of bad food, industrial pollution or a contaminated water supply. Controlling the source stops the outbreak.

Point Source outbreak

Here, all cases appear to occur within one incubation period, suggesting that cases did not arise from person-to-person spread. The fact that the outbreak was of short duration suggests that it was a single, brief (hence "point") exposure that did not persist over time.

Examples include that embarrassing diarrhea saga following the neighbour's summer barbecue, or a respiratory illness in workers following a breakdown of a fume hood.

Point source epidemic curve

Continuing Source outbreak

As with the point source outbreak, a group of people are exposed to a single noxious influence. But here the exposure continues over a longer time (e.g., a contaminated water supply that doesn't get fixed), so the outbreak persists for longer.

The relatively abrupt beginning of the outbreak suggests that many people were exposed simultaneously, rather than it spreading via transmission from one case to another. The fact that no cases arise beyond one incubation period following the termination of the exposure also supports this conclusion.

Continuing source epidemic curve

Intermittent outbreaks

You may occasionally see this pattern. This seems to be a common source that is not well controlled, so outbreaks recur. Depending on the time-frame, it could be seasonal or weather-related, or perhaps due to a common source such as an industrial contaminant being emitted at intervals.

The gaps between the outbreaks might initially suggest person-to-person transmission followed by an incubation period, but this is unlikely because in a transmissible disease the successive peaks would become larger and merge together, as in the next examples.

Graph of an intermittent outbreak pattern
B. Person-to-Person Spread.
Here the disease spreads via person-to-person contact – the classic infectious disease pattern. Controlling the source is no longer sufficient to control the outbreak.   Link: routes of person-to-person transmission.

Index case with limited spread

Here a single 'index' case (for example, a returning traveller) infects other people, and cases arise after an incubation period. (Perhaps confusingly, you may also hear this called a point source with secondary transmission).

The outbreak wanes when the infected people no longer transmit the infection to other susceptible people, perhaps because of successful control measures (isolation or quarantine). The graph suggests this was achieved quite quickly.

Graph showing a point source epidemic curve with an index case and limited spread

Propagated Spread

This begins like an infection from an index case but then develops into a full-blown epidemic with secondary cases infecting new people who, in turn, serve as sources for yet other cases. This produces successively taller peaks, initially separated by one incubation period, but the peaks tend to merge into waves with increasing numbers of cases in each generation. The epidemic continues until the remaining numbers of susceptible individuals declines or until intervention measures take effect.

Epidemic curve showing index case followed by propagated spread