Interpreting and Creating an Epi-Curve

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The overall pattern shown by disease case data when plotted in an epi-curve indicates the possible source of an infection. Knowing the source of the outbreak allows epidemiologists and public health officials to recommend prevention and control measures more quickly. This reduces patient cases and often saves lives.

By the time you finish this lesson, you should be able to:

  • explain the importance of epi-curves in epidemiology
  • describe five types of outbreak epi-curves
  • interpret an epi-curve to identify the possible source of an outbreak and the type of spread


epidemic— numbers of disease cases, injuries, or other health conditions greater than expected in an area or population during a particular period.

epi-curve— a graph that plots the course of an outbreak by time of onset.

histogram— a graph using bars or columns to represent values in a statistical study; in epidemiology, time is usually plotted on the x-axis with number of cases on the y-axis.

incubation period— the time period from exposure to an infectious agent to the onset of symptoms of disease.

index case— the first case of a particular health condition of which health authorities are aware.

outbreak— the sudden rise in the occurrence of a disease or injury in a given area during a given time period; generally considered to be more localized than an epidemic.

transmission—any method by which an infectious agent is spread to a host.

An epi-curve pattern looks different depending on the source of infection and how the disease is transmitted.

Read the different kinds of source outbreaks below and carefully analyze the each example outbreak epi-curve. You will see that by analyzing when the cases have occurred, you can narrow the possibilities for how the outbreak started.

Common source outbreaks are those that arise from a single, shared source. Food contamination, industrial pollution sources, or contaminated water supplies are examples of illnesses which result from common source outbreaks.

  • Point source outbreaks occur when all disease cases seem to occur within one incubation period (the time period from the exposure to pathogen to the onset of symptoms of disease). The point source outbreak below shows that the outbreak was of short duration and did not spread; when the source goes away or is eliminated, the outbreak stopped. This suggests a single (point) of origin and does not suggest person-to-person transmission.


  • Common source with continuing exposure outbreaks occur when cases arise from a single, shared source and the exposure continues over a longer time. The outbreak persists and large numbers of people are exposed simultaneously.


This type of outbreak happens if a large number of people are exposed to a pathogen and the source persists. The epi-curve below indicates that a larger number of people were exposed simultaneously and that the source of the infection was not identified or eliminated quickly; more people became ill. No cases usually occur after the cause of the exposure is eliminated in this type of outbreak.


Looking at the epi-curve below, when would you say the exposure ends?

  • Intermittent outbreak—This type of outbreak shows a common source that does not seem to be well controlled; outbreaks reoccur. The outbreaks could be seasonal or weather-related or result from a common source that is being released into a community at intervals.

 Why would this not be a person-to-person transmission?

 Person-to-person Spread

In contrast to common source outbreaks, person-to-person outbreaks have as many sources as there are people who spread disease.

Index case with limited spread outbreaks occur when an “index case”, sometimes called Patient Zero, infects people who in turn infect more people. The outbreak continues during a limited time frame until there are no more susceptible people to infect or until successful control measures such as isolation or quarantine halts transmission.

The outbreak ends when infected people no longer transmit the disease to other people due to lack of susceptible people or effective control measures.

Propagated spread—This type of outbreak occurs when the outbreak results from an index case but then develops into a much larger (propagated) epidemic. Successively large numbers of cases occur, initially separated by incubation periods.  Numbers of cases begin to merge with increasing cases until the number of susceptible individuals decreases or until control measures are effective.

NOTE: The shape of an outbreak’s epi-curve rarely fits any one type of outbreak perfectly. Epidemiologists must analyze multiple sources of information in order to quickly and accurately pinpoint the source of infection.


  1. What is the source of disease spread in each of the epi-curve below? Explain.

  1. What is the source of disease spread in each of the epi-curve below? Explain.