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Quick-Learn Lesson

Using an Epi Curve to Determine Mode of Spread

This Quick Learn lesson will take approximately 10 minutes to complete.

When you are finished, you will be able to determine the outbreak's likely mode of spread by analyzing an epidemic curve, or “epi curve.”

You can move through this lesson by using the NEXT and BACK icons below.

X and Y Axes

An epidemic curve, or “epi curve,” is a visual display of the onset of illness among cases associated with an outbreak. The epi curve is represented by a graph with two axes that intersect at right angles.

The horizontal x-axis is the date or time of illness onset among cases.

The vertical y-axis is the number of cases.

Each axis is divided into equally spaced intervals, although the intervals for the two axes may differ.

This graph depicts the x and y axis on a epi curve

What an Epi Curve Can Tell You

An epi curve is a visual display of the onset of illness among cases associated with an outbreak.

You can learn a lot about an outbreak from an epi curve, such as

  • Time trend of the outbreak, that is, the distribution of cases over time
  • “Outliers,” or cases that stand apart from the overall pattern
  • General sense of the outbreak's magnitude
  • Inferences about the outbreak's pattern of spread
  • Most likely time of exposure

What an Epi Curve Can Tell You,


The magnitude of an outbreak can be assessed easily with a glance of the epi curve. Are there many cases or just a few?

The time trend, or the distribution of cases over time, will give an indication of where the outbreak is in its course. Are cases still rising or has the outbreak already peaked? Does it appear that the outbreak is over? How long has it been since the last case occurred?

Outliers are cases that stand apart from the other cases. Outliers include the index case, which might be the source of the outbreak, and cases that occur well after other cases, which might indicate secondary spread of the illness.

Magnitude, Time Trend, and Outliers

Below is the epi curve from an outbreak of hepatitis A. If today's date is August 17, what can you conclude about the outbreak?

Select a link below to learn more.

Note: The incubation period for hepatitis A is 25-30 days.

Hepatitis A Cases by Date of Onset in Port Yourtown, Washington, June - August 2010

This graph depicts the onset of Illness among cases of hepatitis A in Port Yourtown, Washington during June to August 2010.

Mode of Spread: Point Source

An epi curve can also be used to make inferences about inferences about an outbreak's most likely mode of spread, suggesting how a disease is transmitted. Transmission occurs in the following ways:

  • Point source
  • Continuous common source
  • Person-to-person spread (propagation)

In a point source outbreak, persons are exposed over a brief time to the same source, such as a single meal or an event. The number of cases rises rapidly to a peak and falls gradually. The majority of cases occur within one incubation period of the disease.

Cryptospordiosis Cases Associated with a Child Care Center by Date of Onset in Port Yourtown, Washington, June 1998

This example shows the typical shape of a point source outbreak.  In this outbreak of cryptosporidiosis (average incubation period = 7 days, range 1-12 days), a toddler was the source of infection for other children and staff at the child care center for one day before being sent home due to diarrhea.  This index case occurred on June 12, about a week before the other cases.  Subsequent cases of cryptosporidiosis began on June 17, rose rapidly over four days, peaked on the 20th, and then declined more gradually.  Most of the cases occurred within one incubation period of cryptosporidiosis.

Mode of Spread: Continuous Common Source

In a continuous common source outbreak, persons are exposed to the same source but exposure is prolonged over a period of days, weeks, or longer. The epi curve rises gradually and might plateau.

Salmonellosis Cases Exposed to Contaminated Salami by Date of Onset, United States, December 2009 – January 2010

In this example, an outbreak of salmonellosis resulted from contaminated salami.  There is a prolonged occurrence of cases as they are exposed to the salami over a two month period, from December 2009 to January 2010.

Mode of Spread: Propagated Outbreak

In a propagated outbreak, there is no common source because the outbreak spreads from person-to-person. The graph will assume the classic epi curve shape of progressively taller peaks, each being one incubation period apart.

Measles Cases by Date of Onset in Aberdeen, South Dakota, October 15, 1970 – January 16, 1971

Example of an outbreak of measles in South Dakota in the early 1970s. It demonstrates the pattern of an epidemic curve associated with person-to-person spread. The first wave of cases began around November 20 and peaked on the 23rd. The initial wave of cases was followed by two subsequent and larger waves peaking on December 4 and December 15th, each about one incubation period after the previous peak.

Analyzing the Mode of Spread

Of course, the shape of an epi curve rarely fits any of these descriptions exactly. For propagated outbreaks, the shape might show overlapping waves of cases that obscure subsequent peaks, and peaks might diminish more slowly over time. You can, however, get a general sense about the mode of spread of an outbreak from its epi curve.

Your Turn: Exercise 1

The average incubation period for Salmonella is 12 to 36 hours and has a range of 6 hours to 10 days.

What do you think is the most likely mode of spread?

Salmonella Enteritidis Gastroenteritis Cases by Date of Onset in Maryland, August 2008

An epi curve depicting a large outbreak of Salmonella Enteritidis infections associated with crab cakes at a church fundraiser in Maryland, 200.

Your Turn: Exercise 2

Let’s look at the epi curve for another outbreak of salmonellosis. Remember, the average incubation period for Salmonella is 12 to 36 hours and has a range of 6 hours to 10 days. What do you think is the most likely mode of spread?

Select the most likely pattern of spread.

Onset of Illness among Cases of Salmonella Typhimurium Infection Associated with Peanut Butter, United States, 2008-2009.

An epi curve depicting the onset of illness among cases of Salmonella Typhimurium infection associated with peanut butter, United States, from September 2008 to January 2009


Now you should be able to analyze an epi curve and determine an outbreak’s mode of spread.

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