# Principles of Epidemiology in Public Health Practice

Epidemiology has quickly risen in global importance during the new coronavirus pandemic. It is crucial to investigate the spread of the new coronavirus as well as finding and mitigating regional outbreaks. It also helps us understand the frequency and occurrences of coronavirus complications that some patients will experience after overcoming the initial acute period of the disease.

Because of its importance, it is also a topic that is currently infested with misinformation on social media. It is thus imperative to spread accurate knowledge about epidemiology, its methods and results online. So what can epidemiology do and what questions can it answer?

Field epidemiologists (also called disease detectives) travel around and investigate outbreaks of infectious diseases around the world from more common occurrences of food-born illnesses to rarer diseases such as Ebola to plague. How to you become a disease detective and investigate outbreaks of infectious diseases? How do you discover an outbreak, collect data during an outbreak and use the data to identify the culprit, treat the patients and stop the outbreak? How do you solve practical matters as a disease detective?

What different kinds of epidemiological data exists and how can they be summarized? How do you measure risk? What is the difference between relative risks and odds ratios? What is an epidemiological association? What does it mean, and perhaps more importantly, what does it not mean? How do you visualize obtain epidemiological data? How can you even measure the impact of public health measures?

Principles of Epidemiology in Public Health Practice is a free textbook in epidemiology from the CDC that also functions as an online course for self-study. It was originally written in 2006, but was updated during 2012. It consists of six different larger categories, from the history of the field and summary statistics to public health surveillance and investigating disease outbreaks. It does not require any prerequisites, but some introductory knowledge about public health and statistics can be useful. However, it is entirely possible for someone who do not have a robust background in public health or statistics to understand the content.

The six larger categories (called lessons) are in turn divided into different sections. Each chapter feature an overview at the start and finishes with a summary, references associated with the lesson, answers to exercises within the lesson and a finally a self-assessment test with answers. Some chapters also have valuable appendices that provides more information.

The first lesson is a basic introduction to the field of epidemiology. It features material on the definitions and history of epidemiology, what the purpose of epidemiology is and how it works. It also wets the appetite by contrasting descriptive and analytic approaches and briefly go over disease occurrence and how diseases work from a person becomes infected to the time the patient either recovers or dies. It also provides basic vocabulary for things like reservoir, different kinds of transmission and so on. This first lesson is, in essence, an introductory overview.

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The second lesson is a bit more mathematical as it explains how to calculate various central tendencies of the data (mean, median, mode and so on) as well as error bars (such as standard deviation and confidence intervals). It goes into some detail about different kinds of variables, distributions and how to pick the right measures for the right purpose.

The third lesson expands by applying the methods learned in the previous lesson to calculate metrics that are directly relevant to epidemiology. This includes raw metrics such as incidence and prevalence as well as metrics that can be directly used to interpret the impact, such as years lost. Information about how to measure association and the real-world impact of public health measures as well as how to interpret these figures is a prominent feature of this lesson. This lesson is centered around risk assessment.

Data visualization in tables and graphs is the major focus of the fourth lesson. It teaches the reader the most crucial information about line graphs, frequency distributions, the dreaded pie charts and box plots.

For those that already master basic mathematical definitions of epidemiological metrics and their practical interpretations, the fifth lesson swiftly ushers the reader into public health surveillance. It provides a step-by-step approach to setting up your own public health surveillance, from deciding what to monitor and how to collect data to how to analyze and interpret the data you collected and interpret it. Finally, the lesson also spends some time on critically analyzing a surveillance program and how to improve it. The appendices in this chapter are vital, as they provide examples of surveillance programs and lists the major health data systems available in the U.S.

Perhaps one of the most exciting lessons is the sixth and final one. Here, the reader gets to learn the basics of how to perform an outbreak investigation in the field in a step-by-step fashion. This is a quite thorough lesson and cover things like field work preparation, discovering outbreaks and lab testing, how to work systematically to find cases and collect epidemiological method. This leads to proposing a hypothesis and evaluating it as well as implementing public health measures to mitigate and prevent the outbreak. A last section describes the often neglected area of how to communicate findings of an outbreak investigation.

This textbook is available both in online format and as a downloadable PDF with the entire textbook. A cached version of the former can be found here that should contain most material. A cached version of the full textbook can be located here.

Debunking Denialism has already covered multiple epidemiology textbooks. The textbook Epidemiology for the Uninitiated focuses more strongly on study design than this one, Basic Epidemiology is more of a full-fledged textbook and Epidemiology and Prevention of Vaccine-Preventable Diseases focuses on vaccines and the diseases they prevent.