Public Health 101 Series 

Instructor name

Title

Organization 

Introduction

to Epidemiology

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2 
• define epidemiology

• describe basic terminology and concepts of epidemiology

• identify types of data sources

• identify basic methods of data collection and 

interpretation

• describe a public health problem in terms of time, place, 

and person

• identify the key components of a descriptive epidemiology 

outbreak investigation

Learning Objectives

After this course, you will be able to

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Topic 1

A Public Health Approach

4 
A Public Health Approach

Surveillance Risk Factor 

Identification

Intervention

Evaluation Implementation

5 
Public Health Core Sciences

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Topic 2

What Is Epidemiology?

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Adapted from: Last JM, ed. A dictionary of epidemiology. 2nd ed. Toronto, Canada: Oxford University Press; 1988.

Epidemiology — Defined

Study of the distribution and 

determinants of health-related 

states among specified 

populations and the application 

of that study to the control of 

health problems

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Epidemiology Purposes

in Public Health Practice

• Discover the agent, host, and environmental factors 

that affect health

• Determine the relative importance of causes of illness, 

disability, and death

• Identify those segments of the population that have the 

greatest risk from specific causes of ill health

• Evaluate the effectiveness of health programs and 

services in improving population health

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Solving Health Problems

Step 1 

Data

collection

Action

Solving health 

problems

Assessment

Hypothesis

testing

Action

Step 2 

Step 3 

Step 4 

Step 1 - 

Surveillance; determine 

time, place, and person

Inference

Determine how and why

Intervention

Step 1 - 

Step 2 

Step 3 

Step 4 

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All of the following illustrate the purpose of 

epidemiology in public health, except

A. identifying populations who are at risk for certain 

diseases. 

B. assessing the effectiveness of interventions.

C. providing treatment for patients in clinical settings.

D. determining the importance of causes of illness

Knowledge Check

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Epidemiologists use a model for studying infectious disease 

and its spread that involves the microbe that causes the 

disease, the organism that harbors the disease, and the 

external factors that cause or allow disease transmission. 

This is also known as

A. host, vector, and transmission.

B. transmission, host, and environment.

C. host, agent, and environment.

D. organism, transmission, and environment.

Knowledge Check

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Epidemiology Key Terms

Topic 3

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Epidemiology Key Terms

epidemic or outbreak: disease occurrence among a population that 

is in excess of what is expected in a given time and place.

cluster: group of cases in a specific time and place that might be 

more than expected. 

endemic: disease or condition present among a population at all 

times.

pandemic: a disease or condition that spreads across regions.

rate: number of cases occurring during a specific period; always 

dependent on the size of the population during that period.

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A. endemic B. pandemic C. epidemic

3. HIV/AIDS is one of the worst global 

diseases in history. It is a/an _________.

B. pandemic

A. endemic

C. epidemic

1. Malaria is present in Africa at all times 

because of the presence of infected 

mosquitoes. Malaria is _____ in Africa.

2. The Ebola virus in parts of Africa is in 

excess of what is expected for this 

region. This virus is a/an ________.

Match each term with the correct example.

Knowledge Check

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In March 1981, an outbreak of measles occurred 

among employees at Factory X in Fort Worth, Texas. 

This group of cases in this specific time and place 

can be described as a ________________. 

A. distribution B. cluster C. determinant

Choose the correct answer.

Knowledge Check

cluster 

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Calculating Rates

Topic 4

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Rates help us compare 

health problems among 

different populations that 

include two or more groups 

who differ by a selected 

characteristic

Comparing Population Characteristics

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Rate Formula

• the number of cases of the illness or condition

• the size of the population at risk

• the period during which we are calculating the rate

To calculate a rate, we first need to determine 

the frequency of disease, which includes 

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Scenario: Unexplained Pneumonia

July 21–24 July 26–Aug 1 August 2

(Morning)

August 2

(Evening)

American Legion 

Convention, 

Philadelphia, 

Pennsylvania

18 deaths 

reported among 

conventioneers

Health care provider at a veterans’ hospital 

in Philadelphia calls CDC to report cases

of severe respiratory illness among attendees 

of the American Legion Convention 

71 additional 

cases reported

Fraser DW, Tsai, T, Orenstein W, et al. Legionnaires’ disease: description of an epidemic of pneumonia. New Engl J Med 

1977;297:1189–97. 20
Fraser DW, Tsai, T, Orenstein W, et al. Legionnaires’ disease: description of an epidemic of pneumonia. New Engl J Med 

1977;297:1189–97.

Legionnaires’ Disease, by Age Group

Age (yrs) Sick Total Percentage 

≥39 3 44 6.8 

40–49 9 160 5.6 

50–59 27 320 8.4 

60–69 12 108 11.1 

≥70 11 54 20.4 

Unknown 0 2 0 

Time: July 21–24, 1976

Frequency Unit size

Hotel A Residents

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Legionnaires’ Disease Rate

Age (yrs) Sick Total Percentage 

≥39 3 44 6.8 

40–49 9 160 5.6 

50–59 27 320 8.4 

60–69 12 108 11.1 

≥70 11 54 20.4 

Unknown 0 2 0 

Frequency Unit Rate

Time: July 21–24, 1976

Hotel A Residents

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Knowledge Check

On Day 1 of a technology conference in San Diego, 15 

presenters who were setting up for their sessions in Annex X 

became ill with flu-like symptoms. During the course of the 

conference, 20 participants who attended sessions in Annex X 

also became ill with the same symptoms.

To begin calculating the rate of this outbreak, investigators should 

first determine

A. the size of the conference population.

B. the number of cases of illness.

C. the number of days the conference was held.

D. the location of the conference.

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Epidemiology Approach 

and Methods

Topic 5

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Epidemiology Study Types

Epidemiology 

study

types

Experimental 

Observational 

Descriptive 

Analytic

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Descriptive and Analytic Epidemiology

Descriptive

epidemiology 

Analytic 

epidemiology 

When was the 

population affected?

How was the 

population affected?

Where was the 

population affected?

Why was the 

population affected?

Who was affected?

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Fatalities Associated with Farm Tractors

In 1982, the number of farm 

tractor-associated deaths 

was described in terms of 

time, place, and person by 

using records from an 

existing surveillance system

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Goodman RA, Smith JD, Sikes RK, et al. Fatalities associated with farm tractor injuries: an epidemiologic study. Public Health Rep 

1985;100:329–33.

Fatalities Associated with Farm Tractors

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Goodman RA, Smith JD, Sikes RK, et al. Fatalities associated with farm tractor injuries: an epidemiologic study. Public Health Rep 

1985;100: 329–33.

Fatalities Associated with Farm Tractors

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Goodman RA, Smith JD, Sikes RK, et al. Fatalities associated with farm tractor injuries: an epidemiologic study. Public Health Rep 

1985;100:329–33.

Fatalities Associated with Farm Tractors

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An epidemiologist is doing a study on 

the sleep patterns of college 

students but does not provide any 

intervention. What type of study is 

this?

A. Qualitative B. Experimental C. Observational

C. Observational

Choose the correct answer from the following choices:

Knowledge Check

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Knowledge Check

A. Descriptive B. Analytic

B. Analytic

A. Descriptive

1. A study of heart disease comparing a 

group who eats healthy foods and 

exercises regularly with one who does 

not in an effort to test association

2. A study to describe the eating habits 

of adolescents aged 13–18 years in 

Community X

Match each term to the correct example below.

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Epidemiology Data Sources 

and Study Design

Topic 6

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Data Sources and Collection Methods

Source Method Example

Individual persons • Questionnaire

• Survey

• Foodborne illness outbreak

• CDC’s National Health and 

Nutrition Examination Survey

• Health data on U.S. residents

Environment • Samples from the 

environment (river 

water, soil)

• Sensors for 

environmental changes

• Collection of water from area 

streams — check for chemical 

pollutants

• Air-quality ratings 

Health care

providers

• Notifications to health 

department if cases of 

certain diseases are 

observed

• Report cases of meningitis to 

health department

Nonhealth–related 

sources (financial, 

legal)

• Sales records

• Court records

• Cigarette sales

• Intoxicated driver arrests

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Studies are conducted in 

an attempt to discover 

associations between an 

exposure or risk factor and 

a health outcome

Conducting Studies

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Study Design — Cross-Sectional Study 

Subjects are selected 

because they are members 

of a certain population 

subset at a certain time

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Study Design — Cohort Study

Subjects are categorized 

on the basis of their 

exposure to one or more 

risk factors

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Study Design Type — Case-Control Study

Subjects identified as 

having a disease or 

condition are compared 

with subjects without the 

same disease or condition

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A. Intoxicated driver arrests.

B. Electronic health records.

C. Measurement of toxins in a river.

D. Medical board action against a physician.

Which of the following are examples of a healthrelated source of data collection?

Knowledge Check

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3. Subjects who have received 

nutritional counseling and who 

have exercised twice a week are 

compared with subjects who have 

not.

A. Cross-Sectional B. Cohort C. Case-Control 

C. Case-Control

B. Cohort

A. Cross-Sectional 

Match each study with the correct definition.

1. Subjects with diabetes are compared 

with subjects without diabetes.

2. A study of women aged 50–60 years 

in a community located close to a 

nuclear power facility.

Knowledge Check

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Investigating an Outbreak

Topic 7

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Outbreak Investigation

• establishing the existence of an 

outbreak

• preparing for fieldwork

• verifying the diagnosis

• defining and identifying cases

• using descriptive epidemiology

• developing hypotheses 

• evaluating the hypotheses

• refining the hypotheses

• implementing control and prevention 

measures

• communicating findings

Ten steps are involved in outbreak investigations, 

including 

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Outbreak Investigation — Steps 1 and 2 

Fraser DW, Tsai, T, Orenstein W, et al. Legionnaires’ disease: description of an epidemic of pneumonia. New Engl J Med 

1977;297 1189–97.

• Use data from data 

sources 

Step 1 — Establishing the 

existence of an outbreak

• Research the disease

• Gather supplies and 

equipment 

• Arrange travel

Step 2 — Preparing for field 

work 

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Outbreak Investigation — Steps 3 and 4 

Fraser DW, Tsai, T, Orenstein W, et al. Legionnaires’ disease: description of an epidemic of pneumonia. New Engl J Med 

1977;297 1189–97.

• Speak with patients

• Review laboratory findings 

and clinical test results

• Establish a case definition 

by using a standard set of 

criteria 

Step 3 — Verifying the 

diagnosis 

Step 4 — Defining and 

identifying cases

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Outbreak Investigation — Step 5

Fraser DW, Tsai, T, Orenstein W, et al. Legionnaires’ disease: description of an epidemic of pneumonia. New Engl J Med 1977;297 1189–97.

Step 5 — Using descriptive 

epidemiology 

• Describe and orient the 

data 

45
Fraser DW, Tsai, T, Orenstein W, et al. Legionnaires’ disease: description of an epidemic of pneumonia. New Engl J Med 

1977;297:1189–97.

Legionnaires’ Disease Cases, by Day

46
Fraser DW, Tsai, T, Orenstein W, et al. Legionnaires’ disease: description of an epidemic of pneumonia. New Engl J Med 

1977;297: 1189–97.

Legionnaires’ Disease Attack Rates

47

Age

(yrs)

Ill Total Percent 

ill

Ill Total Percent

ill

Ill Total Percent 

ill

≤39 3 44 6.8 3 116 2.6 6 160 3.7

40–49 9 160 5.6 11 232 4.7 20 392 5.1

50–59 27 320 8.4 25 523 4.8 52 843 6.2

60–69 12 108 11.1 19 207 9.1 31 315 9.8

≥70 11 54 20.4 5 76 6.5 16 130 12.3

Unknown 0 2 0 0 7 0 0 9 0 

Total 62 688 9.0 63 1,161 5.4 125 1,849 6.8

Hotel A Hotel B Hotel C
Outbreak Investigation — Steps 6, 7, and 8

Fraser DW, Tsai, T, Orenstein W, et al. Legionnaires’ disease: description of an epidemic of pneumonia. New Engl J Med 

1977;297:1189–97.

Step 6 — Develop a focused 

hypothesis 

Step 7 — Evaluate the hypothesis 

for validity

Step 8 — Refine the hypothesis as 

needed 

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Legionnaires’ Disease Study Results

Five months after the first cases 

of Legionnaires’ disease 

occurred, results of the casecontrol study indicated that 

spending time in the lobby of 

Hotel A was a risk factor for 

illness

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Legionnaires’ Disease Study Results

In December 1976, a CDC 

laboratorian successfully located 

the source bacteria after 

continuing to test the specimens 

that were thought to be dead

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Outbreak Investigation — Steps 9 and 10

Fraser DW, Tsai, T, Orenstein W, et al. Legionnaires’ disease: description of an epidemic of pneumonia. New Engl J Med 

1977;297:1189–97.

Step 9 — Implement control and 

prevention measures

• Determine who needs to know

• Determine how information will 

be communicated

• Identify why the information 

needs to be communicated

Step 10 — Communicate findings

• Control and prevent additional 

cases 

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In 1976, during an American Legion Convention, 11 attendees 

had died of apparent heart attacks by August 1. Dr. Campbell 

contacted the Pennsylvania Department of Health after 

realizing he had treated 3 of those 11 attendees. What is the 

first step the Pennsylvania Department of Health should have 

followed?

A. Establish a surveillance system.

B. Alert the convention attendees.

C. Establish the existence of an outbreak.

D. Send an investigation team to the hotel.

Knowledge Check

52
CDC then launched an investigation. However, no effective 

communication existed between scientists in the field 

interviewing patients and those in the laboratory who were 

testing specimens.

As a first step in stopping this outbreak, what should the team 

have done to identify persons who were part of the outbreak?

A. Verify a diagnosis.

B. Establish a case definition to identify cases. 

C. Communicate findings to the public.

D. Implement prevention measures.

Knowledge Check

53
In speculating that the cooling system might be the source 

of the outbreak, what step was the epidemiologist 

implementing?

Knowledge Check

A. Developing a hypothesis.

B. Refining a hypothesis.

C. Evaluating a hypothesis.

D. Verifying a diagnosis.

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In January 1977, the Legionella bacterium was finally identified 

and isolated and was found to be breeding in the cooling 

tower of the hotel’s air-conditioning system; the bacteria then 

spread through the building whenever the system was 

engaged. What should the investigation team do regarding 

their original hypothesis? 

A. Evaluate it.

B. Refine it.

C. Confirm it.

D. Both A and B.

Knowledge Check

55
The finding from this outbreak investigation lead to 

development of new regulations worldwide for climate 

control systems. What step does this illustrate?

A. Communicate the findings.

B. Implement control and prevention measures.

C. Perform descriptive epidemiology.

D. Refine the hypothesis.

Knowledge Check

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During this course, you learned to

Course Summary

• define epidemiology

• describe basic terminology and concepts of epidemiology

• identify types of data sources

• identify basic methods of data collection and 

interpretation

• describe a public health problem in terms of time, place, 

and person

• identify the key components of a descriptive epidemiology 

outbreak investigation

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QUESTIONS? 

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Resources and Additional Reading

• Bogdanich W. Panama releases report on ‘06 poisoning. The New York Times, 

February 14, 

2008.http://www.nytimes.com/2008/02/14/world/americas/14panama.html.

• Centers for Disease Control and Prevention (CDC). CDC helps solve Panama 

mystery illness. Atlanta, GA: US Department of Health and Human Services, CDC 

News, October 7, 2006. http://www.cdc.gov/news/2006_10/panama.htm.

• Dean H. Introduction to public health, epidemiology, and surveillance. Atlanta, GA: 

US Department of Health and Human Services, CDC Science Ambassador 

Program, July 16, 2012.

• Fraser DW, Tsai, T, Orenstein W, et al. Legionnaires’ disease: description of an 

epidemic of pneumonia. New Engl J Med 1977;297:1189–97. 

• Goodman RA, Smith JD, Sikes RK, et al. Fatalities associated with farm tractor 

injuries: an epidemiologic study. Public Health Rep 1985;100:329–33. 

• Centers for Disease Control and Prevention (CDC). How to investigate an outbreak. Epidemiology in the Classroom, November 17, 2004. Atlanta, GA: US Department of 

Health and Human Services, CDC. 

http://www.cdc.gov/excite/classroom/outbreak/steps.htm#step1.

• Last JM, ed. A dictionary of epidemiology. 2nd ed. Toronto, Canada: Oxford 

University Press; 1988.

• Rentz DE, Lewis L, Mujica OJ, et al. Outbreak of acute renal failure in Panama in 

2006: a case-control study. Bull World Health Organ 2008;86:749–56.

• Sakamoto R, Ohno A, Nakahara T, et al. Legionella pneumophilia in rainwater on 

roads. Emerg Infect Dis 2009;15:1295–7. 

• Thacker SB, Birkhead GS. Surveillance [Chapter 3]. In: Gregg, MB, ed. Field 

epidemiology. 3rd ed. New York, NY: Oxford University Press; 2008.

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not imply endorsement by the Division of Scientific Education and Professional 

Development, Center for Surveillance, Epidemiology, and Laboratory Services, 

Centers for Disease Control and Prevention, the Public Health Service, or the U.S. 

Department of Health and Human Services.

The findings and conclusions in this course are those of the authors and do not 

necessarily represent the official position of the Centers for Disease Control and 

Prevention.

Disclaimers 

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Telephone: 1-800-CDC-INFO (232-4636)/TTY: 1-888-232-6348

Visit: http://www.cdc.gov | Contact CDC at: 1-800-CDC-INFO or http://www.cdc.gov/info

The findings and conclusions in this course are those of the authors and do not necessarily represent the official position of the 

Centers for Disease Control and Prevention.

Center for Surveillance, Epidemiology, and Laboratory Services

Division of Scientific Education and Professional Development