Understanding Epidemiology: Key Concepts and Terms Explained

A cornerstone of public health, epidemiology drives our understanding of disease and determines how we respond to or even prevent outbreaks. Many ambitious professionals hope to make a difference by entering this field and contributing to an ever-expanding body of knowledge. That said, it takes years of study to prepare for a career in epidemiology or public health.

One of the earliest and most significant challenges? Adapting to the unique language of epidemiology. This field is built on a wide range of complex terms and concepts that everyday individuals struggle to grasp. Even experienced epidemiologists may need to revisit as emerging research shapes their perception.

A foundational understanding of key epidemiological terms can be transformative, helping epidemiologists carry out important work effectively, plus helping the public better understand risks and preventative strategies. This is a central component of public health education. To that end, we have provided a brief epidemiology glossary, complete with the field's most essential terms.

What Is Epidemiology?

As the study of disease, including how it spreads and how it can be prevented or controlled, epidemiology represents a crucial concentration within the overarching field of public health. This is a data-driven field that reveals patterns in how and why various health conditions occur and which types of people they impact.

Definition and Scope

Epidemiology is a critical branch of medicine and public health that investigates the many factors that play into disease. Britannica explains that this practice "concerns itself with groups of people rather than individual patients and is frequently retrospective, or historical, in nature."

The peer-reviewed journal PLOS ONE describes how definitions of epidemiology have evolved over time. Although largely associated with infectious disease, this practice can involve numerous other health concerns and is therefore shifting toward a broader definition and scope, especially as chronic health issues become more prevalent.

Increasingly, this field integrates the social determinants of health (SDOH), in recognition that the spread of disease and efficacy of control measures align closely with factors such as economic stability and health care access.

Importance to Public Health Education

All professionals involved in public health education should study epidemiology, even if their work involves other practices or specialties. This field determines the extent to which disease affects various populations and can also provide powerful insight into which preventative strategies are most impactful.

Equipped with a strong epidemiological foundation, public health professionals can advocate for and implement research-backed strategies. They can also monitor health trends and convey complex information in a way that guides and empowers communities.

Types of Epidemiology

The public's conception of epidemiology largely involves field epidemiology and infectious disease, and for good reason. When major outbreaks occur, we look to epidemiologists to provide the most up-to-date, research-backed information.

The term "field epidemiology" describes time-sensitive investigations launched in response to urgent health crises, but this is just one of many vital types of epidemiology. Other categories reference the various types of data gathered and analyzed, along with the applications of that data. These essentials are outlined below:

Descriptive Epidemiology

Descriptive epidemiology provides crucial insight into the burden of disease. It offers a valuable framework through which public health coordinators, policymakers, and other stakeholders can understand various health concerns.

Relying on observational methods, descriptive studies often involve case studies and case reports, although cross-sectional studies may also fall into this category. A successful descriptive study will answer basic disease-centered questions such as who, what, when, and where.

Analytical Epidemiology

Revealing the association between disease and exposure, analytical epidemiology relies on a comparative group to test hypotheses about the nature of disease. Specifically, these studies focus on disease determinants, including the link between exposure to disease and outcomes of that exposure. Cohort and case control studies are the most common in this category.

Experimental Epidemiology

While many epidemiological studies are primarily observational, it is possible for epidemiologists to gain greater control over study conditions. Experimental epidemiology accomplishes this with randomized controlled trials (RCTs), which determine the efficacy of new interventions. Participants are randomly assigned to either control groups or experimental groups.

Key Epidemiological Concepts

Through media coverage, a number of people develop some familiarity with critical epidemiological terms and concepts. The most familiar describe the extent to which various diseases or conditions are present in the community, and how these health issues are spread, detected, and treated.

Incidence vs. Prevalence

Frequently confused by the public, incidence and prevalence are among the most important, yet difficult to distinguish concepts in the field of epidemiology. Both represent measures of disease frequency:

• Prevalence. Revealing the number of current cases of a given disease or health condition, the prevalence offers insights into community health status at a given point in time.
  
  
• Incidence. Emphasizing the rate at which new cases occur, the incidence differs considerably from prevalence in that it exclusively focuses on new cases instead of delving into total cases.

Transmission Modes

The term 'transmission' describes how disease passes from one person to the next and eventually, circulates through entire communities. Modes of transmission represent the specific pathways that diseases can take as they're transferred between people. Common transmission modes include:

• Airborne. Typically spread via respiratory droplets, airborne viruses or bacteria can easily spread when infected individuals sneeze, cough, or even laugh. Familiar examples include influenza and COVID-19.
  
  
• Direct contact. Many diseases are primarily spread by body-to-body contact. Conjunctivitis and chickenpox represent common examples of diseases spread largely by direct contact.
  
  
• Waterborne. Caused by contaminated drinking water, or sometimes recreational exposure at pools or beaches, waterborne diseases can be further spread through other forms of contact. Examples include giardia, dysentery, and E. coli.
  
  
• Vector-borne. Insects, especially ticks and mosquitoes, can act as carriers for seveal dangerous diseases, which are often described as vector borne. Lyme disease and West Nile are some of the most well-known and most feared vector-borne diseases.
  
  
• Foodborne. Caused by contamination that may occur during any stage of food production, foodborne diseases are highly preventable, but also highly prevalent. While some may only cause mild discomfort, others can be severe or even life-threatening. Examples include norovirus and salmonella.

Outbreak vs. Epidemic vs. Pandemic

As diseases spread, epidemiologists rely on a few basic terms to reveal how quickly transmission occurs and to what extent the community is vulnerable.

• Outbreak. Typically involving limited geographic areas, outbreaks occur when the number of cases in a specific region rise above expected levels.
  
  
• Epidemic. As diseases move beyond specific locations to include larger regions, epidemiologists transition to the term 'epidemic.' This is larger in scale than an outbreak and may last longer.
  
  
• Pandemic. Used to describe global diseases (or conditions that impact multiple continents), the term 'pandemic' conveys a strong sense of urgency. The most familiar example, of course, is COVID-19, which was declared a pandemic by the World Health Organization (WHO) in March 2020.

Essential Epidemiological Terms

Many people are familiar with basic epidemiological concepts. However, public health professionals must also understand who is most at risk for various diseases and which interventions will be most effective for the community.

Case Definition

During an outbreak investigation, the case definition provides standardized criteria to determine which cases actually qualify as part of the outbreak in question. This definition should describe key elements such as the person impacted by the disease, the place in which the transmission occurred, the time of onset, and clinical features such as symptoms.

Risk Factors

As characteristics associated with an increased risk of infection, risk factors help epidemiologists determine who is most vulnerable to various diseases. A wide variety of variables can be linked to an increased risk of disease. Common examples include age, genetic predisposition, or environmental factors such as air or water pollution.

Measures of Association

Revealing the link between exposure and outcomes, such as infection, measures of association are important metrics because they quantify associations while helping epidemiologists prioritize risk factors. Examples include:

• Relative Risk (RR). Also known as the risk ratio, this metric compares the risk of a particular disease between various groups of people. A ratio of 1.0 suggests that both groups are equally vulnerable, while a ratio higher than 1.0 suggests that the exposed group has a larger risk. It is also possible for the RR to drop below 1.0.
  
  
• Odds Ratio (OR). Comparing exposures to outcomes, this ratio can function as an estimate of the previously described risk ratio. This reveals the likelihood of exposures leading to disease. This metric is often used in case-control studies.
  
  
• Attributable Risk (AR). Demonstrating the extent to which disease risk relates to exposure, the attributable risk helps epidemiologists understand how individual risk factors play into the overarching incidence of disease.

Public Health Interventions

Public health interventions determine how outbreaks, both current and future, are addressed on a wide scale. Common terms describing interventions focus on when and how outbreaks are addressed:

• Prevention. Proactive in nature, prevention addresses the main risk factors associated with the development of outbreaks. The public often confuses this concept with control, but prevention focuses on reducing the likelihood of an outbreak occurring in the first place. Common examples include vaccination and needle exchange systems. Some measures, like hand hygiene, relate to both prevention and control.
  
  
• Control. As previously discussed, parameters such as prevalence and incidence are revealed, control measures can help limit the further spread of disease. Depending on the scope or severity of the outbreak, this could include contact tracing, isolation, or even quarantine. Control may also require educational efforts so that the public is more inclined to reduce exposure through hand hygiene or by wearing face masks.
  
  
• Treatment. Once individuals have developed diseases or health conditions, treatment determines which measures are implemented to reduce the severity of symptoms. Depending on the situation, this may involve medication, surgical treatments, or nutritional support.

Other Epidemiological Terms to Know

We have already covered several fundamental epidemiology concepts, but many other terms can further influence how epidemiologists and public health professionals understand or address outbreaks. Additional terms worth discussing include:

• Mortality. The rate of mortality can be calculated by determining the number of deaths attributed to a particular disease or condition, then dividing that figure by the total population. Epidemiology typically focuses on cause-specific mortality rates.
  
  
• Morbidity. Although it sounds similar to 'mortality,' the term 'morbidity' focuses more on the incidence and prevalence of disease, regardless of whether that disease is deemed deadly. The National Cancer Institute defines this as "the amount of disease within a population."
  
  
• Endemic. As illnesses evolve and become continually present, they are referred to as endemic. To qualify as endemic, disease prevalence must remain reasonably stable over time, although seasonal spikes may occur. Returning to the familiar example of COVID-19, a wealth of researchers now assume that this disease has reached an endemic state.
  
  
• Virulence. This term describes the ability of a given pathogen to overcome defenses in the immune system and cause damage to the host. This determines the extent to which various pathogens can harm individuals and communities.
  
  
• Carrier. Not all those exposed to diseases will show symptoms. However, some may act as carriers, helping to spread diseases even when their own infections are impossible to detect.

Find Your Future in Public Health With Texas A&M

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