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CHAPTER 17: Investigating the Types of Epidemiologic Studies
Introduction
As a beginner in public health, you may be on a quest to know about diseases that are affecting your community, other countries, and the world. If you desire to build a career in a field of epidemiology, you want to know more about what causes certain diseases and how they’re transmitted, or as a public health professional, you want to advise people about disease prevention.
You’ve come to the right place. Epidemiology has been a hot topic in the past few years with the Covid-19 pandemic, but there’s so much more to it. Getting a degree in epidemiology is a good choice because the concepts and skills of epidemiology will prepare you for plenty of jobs in public health. Even if you aren’t a researcher or a data analyst, epidemiology can help you get a sense about numbers when you hear that the Covid-19 rates are rising or coming down, or when you hear that Forest County of Hattiesburg, Mississippi is the hot spot of lead poisoning.
About This Book
Epidemiology For Dummies emerged from the needs of undergraduate and graduate students in public health, especially in the field of epidemiology. During the Covid-19 pandemic, almost all face-to-face classes were closed and classes were only offered online. Some students faced challenges in fully understanding some difficult topics during this online format.
This handy guide isn’t a textbook or workbook in epidemiology. Rather, I wrote this book based on my decades of experience in practicing medicine, conducting health research, and teaching public health for undergraduate as well as graduate students to help explain the concepts of epidemiology in plain English with plenty of real-life examples, calculations, and illustrations.
Here you can read about an array of concepts, starting from Epidemiology 101 to more advanced research methods to ethics in conducting human research. I focus on the following areas:
» The history of the development of public health and epidemiology
» The epidemiologic triangle
» Person-place-time distribution of diseases
» Causal association and Hill’s criteria
» The three levels of prevention
» Vaccine-preventable diseases
» Disease surveillance
» Steps of outbreak investigations
» Screening methods
» Epidemiologic study designs
» Bias and confounding
» Population projection
» Ethics in human research
I also provide step-by-step explanations and answers to practical issues like the following:
» Investigating outbreaks and analyzing data
» Solving problems of sensitivity, specificity, and predictive value
» Calculating commonly used rates, ratios, and proportions
» Calculating incidence, prevalence, and standardized mortality ratio (SMR)
» Calculating country-level data for population projection
Foolish Assumptions
When writing this book, I’ve made a few assumptions about you, my dear reader. I made the following assumptions:
» You’re bored or somewhat disappointed by reading textbooks with small fonts, full of concepts after concepts, with few examples of topics that don’t provide real-life examples of epidemiologic applications.
» You’ve struggled understanding some of the technical terms and concepts in epidemiology, but you see your future working in public health.
» You’re excited about applying what you’ve studied in your epidemiology courses, but you need a little extra help with the calculations.
» You’ve been working a few years in a health department and you’ve faced many issues, such as choosing the right study design based on your resources, preparing a questionnaire, or investigating and controlling an epidemic.
» You’re a silent learner, and you don’t ask questions in a class.
Icons Used in This Book
Throughout this book, you can find icons — small pictures next to the text that point out extra-important information. Here’s what they all mean:
For gems of accumulated wisdom — quite often the kind learned by painful experience! — follow this icon.
Consider this icon like a stop sign. When you see it, stop and pay extra attention because you might make a mistake — perhaps in a math calculation or something extra important — if you’re not careful enough.
You’re trying to do things correctly and efficiently. Problem is, you may not always know what’s right and what isn’t. When you see this icon, pay attention to the text.
This icon is used for more advanced material that you don’t need to read to understand the concept at hand. It’s information that’s interesting but not absolutely essential.
This icon points out concepts with practical examples, some from my own research.
Beyond This Book
This book is chock-full of tips and other pieces of helpful advice you can use as you study epidemiology. I provide links where you can go online for more information. In addition, check out the book’s Cheat Sheet at www.dummies.com and search for “Epidemiology For Dummies Cheat Sheet” for information to reference on a regular basis.
Where to Go from Here
This book represents a starting point for concepts and uses of epidemiology. Your new learning curve in public health is just beginning. So, now what? You can flip through the Index or Table of Contents to find a subject that interests you.
Or you can turn to whatever section looks to have the answers and information you’re wanting most. No matter where you start, you can read a section or two, stop, and then come back when you need more guidance. I tell my students, “Epidemiology is easier than you think and more fun than you can imagine.”
1 Getting Started with Epidemiology
IN THIS PART . . .
Gain a basic knowledge about infection and infecting agents such as bacteria, viruses, parasites, yeasts, molds, and others; how diseases occur, and why you are not sick all the time despite living in a world with so many infecting agents.
Get background information about how the science of modern-day epidemiology came into play through different stages of legendary works in the field.
Understand the scope of epidemiology, two major functions of epidemiology, and the importance of epidemiology in measuring health status, searching for disease causation, and controlling and preventing diseases and events in humans.
Identify sources of epidemiologic data such as the Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO), census data, vital statistics, and others.
Explore older and modern theories of diseases causation and important contributions of people in laying foundation and the development of different branches of public health.
Uncover milestones in public health, such as James Lind’s study of finding the treatment for scurvy, the cholera investigation of John Snow, Joseph Goldberger’s study of the cause of pellagra, the famous influenza pandemic, the eradication of smallpox, the connection between smoking and cancer, the development of theories of causal association, and more.
Recognize the means and ways of controlling several common infections, such as waterborne diseases, airborne diseases, vector-borne diseases, parasitic diseases, and sexually transmitted infections.
IN THIS CHAPTER
» Introducing the concepts of epidemiology
» Finding out about uses of numbers
» Focusing on prevention
Chapter 1
Entering the World of Epidemiology
You’re about to enter the wonderful world of epidemiology — an adventure of hunting for five million trillion trillion (that’s a five with 30 zeroes after it) bacteria, about six million parasites, and nearly 7,000 virus species that are prevailing in the world. Thank goodness not all of them are harmful to you. The vast majority of them either live on the planet or inside you harmlessly or keep at bay like microscopic superheroes. Fewer than 100 species of bacteria, 300 species of parasitic worms, about 70 species of protozoa, and more than 200 viruses are known to cause disease in humans.
Most of the disease agents, which are infectious in nature, are controllable, either by antibiotics, vaccines, or by other public health preventive measures, such as personal hygiene, safe water supply, proper sanitation, healthy food habits, and by improving your resistance to infecting agents.
This chapter gives you an overview of this world of epidemiology and serves as a jumping-off point into this book. This chapter previews the concepts of epidemiology, mentions the importance of crunching numbers, addresses disease prevention, and discusses disease prevention, and more.
Introducing Epidemiology
Epidemiology is the study of human diseases and events. Epidemiologists are disease detectives whose jobs include the following:
» Searching for the cause of diseases in humans: All associations aren’t causal. Chapter 16 provides concepts on knowing if an association between an exposure and a disease is causal or not.
» Identifying people who are at risk: Certain host factors are associated with diseases. Descriptive epidemiology (see Chapter 7) deals with person, place, and time factors that are associated with diseases. Chapter 5 addresses the risk of people in getting different types of diseases.
» Determining how to control or stop the spread: Knowing the chain of disease transmission helps you prevent or control the spread of a disease. Refer to Chapter 5 for more information about chain of disease transmission.
» Preventing the disease from happening again: Chapter 11 explains the different levels of disease prevention with practical examples.
Recognizing How Numbers Can Help Study Disease
Epidemiology and biostatistics are like cousin sisters. However, epidemiology isn’t learning about math. I often ask my students whether they like math, and most of the time, they respond no and sometimes emphatically that they hate math. That’s okay. Epidemiology doesn’t deal with hard-core math problems. These sections explain that epidemiologists use a basic knowledge of algebra to calculate numbers.
Grappling with the epidemiologic triangle
The concept of the epidemiologic triangle includes these three factors of a disease:
» Agent: The causative factor (such as a bacteria, virus, or parasite). In other words, the what that causes the disease.
» Host: Humans and non-human animals can harbor a disease. They’re called disease hosts.
» Environment: Factors in the environment such as temperature (hot or cold), noise, moisture, dusts, and others cause diseases. Also, agents and hosts both live in the environment, which makes a balance between disease and health.
When germs enter and grow in the human body, it’s called an infection. The germs may be bacteria, viruses, parasites, yeasts, fungi, or other microorganisms. They’re agents for an infectious disease. These agents live and multiply in the environment that humans live. But an infection doesn’t necessarily lead to a disease. The favorable conditions in the environment help agents grow.
On the other hand, when a person’s immune system is strong enough, it can fight the germs and cure an infection without causing a disease. If immunity is low, the germ gets the upper hand, and the person fails to resist the infection, which in turn leads to a disease. In a chronic disease model, as I describe in Chapter 6, you can find that the causes are multifactorial — they are called risk factors, instead of agents. Chapter 16 explains the concept of multiple risk factors for a noncommunicable disease.
Classifying epidemiology
Two broad classifications of epidemiology that you need to know are as follows:
» Descriptive: Descriptive epidemiology provides you answers for what, when, where, and who questions. Most health surveys, censuses, and case reports are descriptive in nature. In descriptive studies, you can identify risk groups or hot spots (or areas where diseases and agents cluster). Descriptive information can be highly valuable in generating a hypothesis and conducting a future study to evaluate the hypothesis through experimental studies, interventional studies, or a randomized controlled clinical trial (RCT) (you can find details in Chapter 17).
» Analytical: Analytical epidemiology deals with the why and how questions. Some statistical tests (called inferential statistics) are used for answering these questions. Analytical epidemiology is used to prove the hypothesis.
Understanding epidemiologic transition
The changing nature of diseases is a continuous process, and it depends on several factors including the ecology, public health measures, vaccine development, antibiotic use, genetics, and other host factors. The transition of disease occurrence from acute and infectious diseases to chronic and noncommunicable diseases (NCDs) is called the epidemiologic transition. Chapter 8 describes in greater detail this changing pattern of diseases.
Consider the following: The worldwide pandemic of Covid-19 has evolved as one of the most fatal diseases in human history. Similarly, a few other pandemics including plague, influenza (flu), smallpox, and HIV/AIDS have caused devastations and killed a large number of people. Some other infectious diseases such as pneumonia, diarrhea, malaria, and tuberculosis are still common causes of morbidity and mortality in many developing countries.
On the other hand, noncommunicable diseases such as heart disease, cancer, stroke, unintentional injuries, chronic obstructive pulmonary disease (COPD), Alzheimer’s disease, and diabetes are the leading causes of death in the United States. Developed countries have curtailed mortality rates from infectious diseases. Some of the infectious disease has been eliminated (such as smallpox) in the world or reduced to a minimum level (such as polio, tetanus, measles) in developed countries. Polio is expected to be eliminated soon.
Connecting demography and disease
Changes in demography are certainly affecting the disease pattern and the healthcare costs. Health consequences of aging are many, including pain and arthritis, osteoporosis, falls and accidents, hearing defects, eye problems, heart disease, diabetes, depression, Alzheimer’s, and senile dementia.
The burden of healthcare costs is also escalating. For example, in recent years, one-fifth of older Americans spent more than $2,000 out of pocket on healthcare. Chapter 9 looks at different demographics, including a comparative picture of the population structure of several countries, a list of the ten most populous states in the United States, and the top ten countries with the largest proportion of senior citizens. In addition, you can discover how to project step by step the future population of several countries.
Figuring out rates and risks
One of the focuses of descriptive epidemiology is to calculate rates and risks. Epidemiologists summarize health reports and describe the risks based on numbers. Chapter 10 explains how to calculate important rates such as crude birth rate, crude death rate, age-specific rates such as infant mortality rate, neonatal mortality rate, post-neonatal mortality rate, and perinatal mortality rate, causespecific rates such as cancer- and heart-diseases mortality rate, and genderspecific rates such as breast cancer rates and prostate cancer rates.
