# Introduction

An outbreak is essentially the same thing as an epidemic, i.e., an increased frequency of a disease above the usual rate (endemic rate) in a given population or geographic area. Pandemic refers to simultaneous epidemics occurring in multiple locations across the globe. Traditionally, these terms referred to infectious diseases, but they can also be used to describe non-infectious diseases and chronic conditions, such as lung cancer and obesity. In addition, the principles of investigation are similar for all of these. This module provides a practical introduction to the steps involved in outbreak investigations, and it provides some useful tools.

# Learning Objectives

After successfully completing this section, the student will be able to:

- Define the terms outbreak, epidemic, endemic, and pandemic.
- List the steps in the investigation of an outbreak.
- Given the initial information of a possible disease outbreak, describe how to determine whether an epidemic exists.
- Describe the importance of having a case definition and the factors to consider in developing a case definition.
- Explain how to gather, record, and analyze descriptive data related to characteristics of person, place, and time that will generate hypotheses about the source of an outbreak.
- Define the primary difference between descriptive studies and analytical studies.
- Create a "line listing" using an Excel spreadsheet.
- Define and calculate prevalence and incidence.
- Define and calculate a) mortality rate, b) morbidity rate, c) attack rate, d) case-fatality rate.
- Perform basic functions in Excel, including:

- Labeling columns and rows & entering text and numeric data.

- Sorting data.

- Using Excel functions to tabulate data using the COUNT and SUM functions to tabulate data.

- Identify the following types of epidemic curves: a) point source epidemic, b) continuous source epidemic, and c) propagated source epidemic.
- Distinguish between cohort studies and case-control studies, be able to describe their key features, and be able to give an example of each.
- Calculate and interpret a risk ratio for a cohort study.
- Calculate and interpret an odds ratio for a case-control study.
- Demonstrate how to use the provided Excel worksheets to perform a statistical analysis for either a cohort study or a case-control study, including calculation and interpretation of p-values and 95% confidence intervals.

**A Salmonella Outbreak after a School Luncheon – A Cohort Study**

- Use the Excel skills outlined above to analyze the line listing data to identify the food that was responsible.
- Calculate and interpret relative risks, p-values, and 95% confidence intervals.
- Interpret your analysis of the outbreak and discuss how your analysis relates to the official report of the study conducted by the Massachusetts Department of Public Health.
- Discuss how you would explain these results to concerned citizens who had no knowledge of epidemiology.

**The Hepatitis A Outbreak in Marshfield, MA – A Case-Control Study**

- Create and interpret the epidemic curve for the hepatitis A outbreak.
- Use the Excel skills outlined above to analyze the line listing data from the hepatitis A outbreak, including odds ratios, p-values, and 95% confidence intervals.
- Interpret your analysis of the hepatitis outbreak and discuss how your analysis relates to the official report of the study conducted by the Massachusetts Department of Public Health.
- Discuss how you would explain these results to concerned citizens who had no knowledge of epidemiology.