# Lesson 6: Investigating an Outbreak

This is an online version of a printed textbook. It is not intended to be an online course.

Refer to the book or to the electronic PDF version (511 pages) for printable versions of text, figures, and tables.

## Overview

Public health department staff responsible for reviewing disease report forms notice that the number of forms for shigellosis seems higher than usual this week. Someone from a nursing home calls to report several cases of pneumonia among its residents. Is the number of cases in either of these situations actually higher than usual? What should be used to estimate “usual?” If it is higher than usual, should the health department staff call the situation a cluster, an outbreak, an epidemic? Is a field investigation needed? What criteria should they use to decide? And if they decide that a field investigation is indeed warranted, how do they go about conducting such an investigation? These and related questions will be addressed in this lesson.

## Objectives

After studying this lesson and answering the questions in the exercises, you will be able to:

- List the reasons that health agencies investigate reported outbreaks
- List the steps in the investigation of an outbreak
- Define cluster, outbreak, and epidemic
- Given the initial information of a possible disease outbreak, describe how to determine whether an epidemic exists
- State the purpose of a line listing
- Given information about a community outbreak of disease, list the initial steps of an investigation
- Given the appropriate information from the initial steps of an outbreak investigation, develop biologically plausible hypotheses
- Draw and interpret an epidemic curve
- Given data in a two-by-two table, calculate the appropriate measure of association