What is epidemiology?

A woman draws a graph with an upward trend on a whiteboard.
Epidemiologists study the emergence, distribution and control of disease, disability and death among groups of people. (Image credit: Shutterstock)

Epidemiology is the study of the emergence, distribution and control of disease, disability and death among groups of people. The field of epidemiology combines the sciences of biology, clinical medicine, sociology, mathematics and ecology to understand patterns of health problems and improve human health across the globe.

"Epidemiology is a tool, in many ways, to understand the distribution of disease in populations, and the factors that lead to higher or lower rates of disease and ways of effectively preventing disease," Lewis Kuller, a professor of epidemiology at the University of Pittsburgh Graduate School of Public Health, said in a 2010 interview with the journal Epidemiology.

What's an epidemiologist? 

The word "epidemiology" is based on the Greek words "epi," which means "upon" or "befall," and "demos," which means "the people." So, if taken literally, epidemiology is the study of what befalls the people. The ancient Greek physician Hippocrates is considered the father of epidemiology. Living between the fourth and fifth centuries B.C., when most treatments and medicine relied on superstition, Hippocrates was the first to use rational thinking to attribute health problems to environmental or natural causes. He suggested treatments such as surgery, dietary modifications and herbal remedies. He also coined the terms "endemic" (a disease specific to a certain area) and "epidemic" (a disease specific to a point in time) — words epidemiologists (those who study or practice epidemiology) use to this day. 

Related: What is a pandemic? 

Epidemiologists can be thought of as doctors of the masses. A clinical physician is different from an epidemiologist because the physician focuses on the health and well-being of individual patients, and diagnoses and treats each patient based on clinical judgment, experience and scientific knowledge. An epidemiologist, on the other hand, focuses on the collective health of communities and sometimes global populations of people. They try to understand the source of disease and other negative health effects, and estimate how many people are exposed and how the disease might spread through a population. Epidemiologists use statistics, mathematical and biological models and scientific knowledge to determine appropriate public health measures to control and prevent health problems.

"What we do, basically, is to try to understand the [cause] of disease, the places where we can do prevention, and then test whether we can prevent disease," Kuller said.

Although generally associated with infectious diseases and outbreaks, such as the COVID-19 pandemic, epidemiologists can also study noninfectious health problems, such as the prevalence of lung cancer from smoking or a community's increase in homicide rates. Although the causes of these health problems may be different, many of the tools epidemiologists use to investigate these issues are the same.

The epidemiologic triangle

As a disease detective, every epidemiologist needs an extensive toolbox to find and investigate clues. And at the center of every epidemiologist's toolbox is the so-called epidemiologic triangle. 

The triangle is a model for explaining the connection between the cause of a disease and the conditions that allow it to reproduce or spread. The triangle has three corners representing the "who, what and where" of any good question. The "who" is the host, or the person who has the disease. The "what" is the agent, or the cause of the disease. And the "where" is the environment, or the external factors that allow and aid in the transmission of the disease. 

The goal of an epidemiologist is to uncover information that can help sever at least one link between the corners of the triangle, breaking the connection between the host, agent and environment, and stopping the disease in its tracks.

"In the old days, if a physician found the bug that made people get sick, some might have thought that the problem was solved," Rebecca Prevots, an epidemiologist at the National Institute of Allergy and Infectious Diseases, said in an interview with Infectious Diseases Hub. "Now we know that just finding the organism is not just enough — epidemiology is an integrative discipline, and we need to understand how the host and the environment interact to cause disease. Through using advanced study design and analytic techniques, epidemiologists can use information both about the humans, or hosts, including genetic risk factors, and the organisms."

Epidemiology and the coronavirus pandemic

In March 2020, during the COVID-19 pandemic, epidemiologist Neil Ferguson and fellow researchers at Imperial College London released a mathematical model detailing the potential impact of the disease if various prevention methods were implemented or not. The novel coronavirus, which started in Wuhan, China, quickly spread across the globe and overwhelmed hospital beds with patients suffering from acute respiratory problems and other complications. 

"We use the latest estimates of severity to show that policy strategies which aim to mitigate the epidemic might halve deaths and reduce peak healthcare demand by two-thirds but that this will not be enough to prevent health systems being overwhelmed," Ferguson said in a statement.

Ferguson and his team went on to recommend "more intensive and socially disruptive interventions," such as large-scale social distancing, to slow the transmission of the virus and prevent the deaths of millions. His team's recommendations became public policy in many countries in the hope of flattening the exponential growth of new cases.

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