Chapter Four: Epidemiology
Overview
Epidemiology is the study of the distribution and determinants of health and disease in human populations. It is the principal science of public health, and its use is essential in disease surveillance. The purpose of epidemiology is to:
- 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
Epidemiology approaches disease surveillance from a public health perspective using questions such as these:
- What is the problem? (surveillance)
- What are the causes? (risk factor identification)
- What is the solution? (intervention evaluation)
- How is the solution used? (implementation)
Learning Objectives
- Define epidemiology
- Describe how epidemiology is used to solve health problems
- Describe basic terminology and concepts of epidemiology
- Differentiate between incidence and prevalence
- Understand epidemiology in terms of levels of prevention
Key Terms
- epidemiology
rate, prevalence, and incidence
mortality and morbidity
epidemic, endemic, pandemic, and cluster
notifiable conditions
Content in this chapter was adapted from the Centers for Disease Control and Prevention (2018) and the HSC Public Health Agency (Arnold et al., 2020).
Introduction
Nurses use epidemiology to better understand patterns of disease and treatment options for the patients they serve. Knowledge of epidemiology can help health care professionals set priorities for health education programs, use health resources more effectively, and plan strategies to help meet new health needs. Community and public health nurses have played an integral role in identifying diseases and the organisms that have caused them. Nurses must use what they learn during investigation to make recommendations to control disease spread or prevent a future occurrence.
What is Epidemiology?
Epidemiology is the study of the distribution and determinants of health and disease in human populations. It is the principal science of public health, and its use is essential in disease surveillance. Epidemiology is the method used to find the causes of health outcomes and diseases in populations. In epidemiology, the patient is the community, and individuals are viewed collectively. By definition, epidemiology is the study (scientific, systematic, and data driven) of the distribution (frequency, pattern) and determinants (causes, risk factors) of health-related states and events (not just diseases) in specified populations (neighborhood, school, city, state, country, global).
The purpose of epidemiology is to:
- 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
As Figure 4.1 demonstrates, epidemiology approaches the study of diseases in human populations from a public health perspective using questions such as these:
- What is the problem?
- Surveillance
- What is the cause?
- Risk factor identification
- What works?
- Intervention evaluation
- How is it used?
- Implementation
Long Description for Figure 4.1
What is the problem? Surveillance stage. What is the cause? Risk factor identification stage. What works? Intervention evaluation stage. How do you do it? Implementation stage.
Like investigators at the scene of a crime, community health nurses will begin by looking for clues. They will systematically gather information, asking questions such as these:
- Who is sick?
- What are their symptoms?
- When did they get sick?
- Where could they have been exposed?
Using statistical analysis, the nurse and other community partners will study answers to these questions to discover how a particular health problem was introduced (Centers for Disease Control and Prevention [CDC], 2016).
The types of public health problems that are scientifically investigated using epidemiological methods include environmental exposures to lead, heavy metals, and air pollutants; infectious diseases from influenza and foodborne illness; injuries from homicides and mass casualties; noninfectious diseases such as birth defects and cancer; chronic diseases such as diabetes; natural disasters such as Hurricane Katrina and the 2010 Haiti earthquake; and the terrorist acts of 9/11.
Epidemiologic data can also help with social justice initiatives to determine if communities or groups of people are at risk for certain diseases or health outcomes and help find ways to bring needed resources to at-risk groups and communities. Epidemiological research can help us to understand measures that will prevent diseases, such as vaccines (primary prevention), and help us to find screening tools and effective early treatment/cures for disease (secondary prevention), or even suggest strategies for the care of patients with chronic advanced disease (tertiary prevention).
Key Epidemiology Terms to Know
There are certain terms to know for understanding epidemiology. An outbreak or an epidemic exists when there are more cases of a particular disease than expected in a given area or among a specific group of people over a particular period. Another term is endemic, used when a population has a high level of the disease all the time. For example, malaria is endemic in parts of Africa. As we know all too well, a pandemic is a disease or condition that spreads globally, such as COVID-19. A cluster is a group of cases in a specific time and place that exceeds what is expected.
To understand epidemiology, rates also need to be understood. In epidemiology, a rate is a measure of the frequency with which an event occurs in a defined population over a specified time. Rates are generally reported as percentages so that disease frequency can be compared in different locations, at different times, or among different groups of people with different-sized populations.
Incidence rate and prevalence rate (incidence and prevalence) are important terms. The incidence rate is the rate of occurrences of new cases of a disease in a certain period. The prevalence rate is the number of cases of a disease in the population at a given time. The prevalence rate contains new and existing cases, whereas the incidence rate is looking only at the new cases. (Remember this by thinking that the ‘inc” at the beginning of “incidence” stands for In terms of New Cases, or INC.)
The terms morbidity and mortality rates are also important. Morbidity refers to the rate of illness in a community or population, and mortality rates are the rates of death in a specific community or population.
Understanding Incidence and Prevalence
One way to understand the difference between incidence and prevalence is to think of water entering and leaving a tub (Figure 4.2).
Long Description for Figure [X]
Water entering the bathtub is labeled incidence. Water in the bathtub is labeled prevalence. Water leaving the bathtub when the plug is released is death or mortality. Evaporation is labeled recovery.
Incidence
Incidence measures new cases of a disease in the population during a specified period and helps to identify increasing or decreasing transmission rates.
In the bathtub, the incidence is represented by the drops of water entering the bath from the faucet. Think of each drop of water as an increase in the incidence. Understanding a disease’s incidence can influence decision-making about public health interventions necessary to slow transmission rates (Arnold et al., 2020).
Prevalence
Prevalence includes both new and existing cases of the disease during a specified period.
In the bathtub, prevalence is represented by the total amount of water at a specific point in time. As we all know, this quantity can change at any time. We can add more water by turning on the faucet. We can also lose water through evaporation or by opening the drain. Therefore, the filled bath represents the total number of cases at a point in time (the prevalence). Arnold et. al. (2020) explain that prevalence is a reflection of a number of issues: “the number of new cases (turning on the tap), the speed of recovery—which is influenced by disease severity, treatment options (evaporation), and the number of people that lose their lives to the disease (releasing the plug).”
Incidence and prevalence are both necessary to better understand a disease process, and both are used to guide public health decisions.
Notifiable Conditions
Content in this section was adapted from Foundations of Epidemiology, by M. L. Bovbjerg (2020).
There is a list of conditions—mostly infectious diseases, but a few chronic diseases and injuries also make the list—that must be reported to the CDC whenever they are encountered by clinicians or health department officials. For example, say a patient presents to a primary care clinic complaining of high fever, cough, and watery eyes followed by a full-body rash. The nurse practitioner who sees the patient diagnoses measles. This clinic must then report the measles case to the local health department, who in turn reports it to the state health department, who in turn reports it to the CDC. This reporting ideally happens quickly, in a matter of days (or within hours for a potentially major threat). The list of nationally notifiable conditions is reviewed every year or so and revised according to current public health threats and priorities.
For instance, the Zika virus and its associated congenital conditions were added to the list in 2016. Some of the conditions on the list are extremely rare (human rabies, plague) or have even been eradicated (smallpox). However, they remain on the notifiable conditions list because in these cases, our expected level (also called the endemic level) is 0, and these conditions are dangerous enough that even one suspected case would be cause for an immediate public health intervention (Bovbjerg, 2020).
What is the Problem?
Surveillance and the Epidemiologic Triangle
Content in this section was adapted from Healthy Schools BAM! Body and Mind Classroom Resources for Teachers Program, “Lesson 1: Understanding the Epidemiologic Triangle Through Infectious Disease,” from the Centers for Disease Control and Prevention (2015).
The epidemiologic triangle (Figure 4.3) is a model that scientists have developed for studying health problems and conducting surveillance. The triangle has three corners (called vertices):
- Agent, or microbe that causes the disease (the “what” of the triangle)
- Host, or organism harboring the disease (the “who” of the triangle)
- Environment, or those external factors that cause or allow disease transmission (the “where” of the triangle)
All three corners need to be present for diseases to occur. The epidemiologist aims to develop a plan that will break at least one of the sides of the triangle, disrupting the connection between the environment, the host, and the agent to stop the continuation of the disease (CDC, 2015).
To break one side of the epidemiologic triangle, information is needed regarding the disease. The process for this is called surveillance, which is an ongoing systematic collection, analysis, and interpretation of health-related data essential to planning, implementation, and evaluation of public health practice (CDC 2022a). The information collected through the process of surveillance is disseminated to those responsible for prevention and control of the identified health problem. Surveillance is foundational for public health practice because it helps us to understand diseases and their spread. The data can be found through local health departments, state health departments, and the CDC.
Healthy People 2030 and Epidemiology
Content in this section was adapted from the Office of Disease Prevention and Health Promotion (2022).
Community and public health infrastructure provide the necessary foundation for all public health services, from vaccinations to chronic disease prevention programs to emergency preparedness efforts. Community health nurses must be well versed in collecting and interpreting epidemiological data to ensure a strong public health infrastructure.
A strong public health infrastructure includes a capable and qualified workforce, up-to-date data and information systems, and agencies that can assess and respond to public health needs. While a strong infrastructure depends on many organizations working together, public health departments play a central role in the nation’s public health system. Federal agencies rely on solid public health infrastructure in state, tribal, local, and territorial jurisdictions. Community health nurses will play a vital role in meeting Healthy People 2030 goals.
Healthy People 2030 community goals:
- Increase the proportion of local jurisdictions that have a health improvement plan
- Increase the proportion of state and territorial jurisdictions that have a health improvement plan
- Increase the proportion of tribal communities that have a health improvement plan
- Explore the impact of community health assessment and improvement planning efforts
- Explore the impact of public health accreditation and national standards (ODPHP, 2022)
The Nurse’s Role in Epidemiology
Public health nurses provide a critical linkage between epidemiological data and clinical understanding of health and illness as it is experienced in people’s lives. This understanding is translated into action for the public good. An illustration of this role is the surveillance and monitoring of disease trends within the community. Emerging patterns that potentially threaten the public’s health are identified and appropriate interventions planned, coordinated, and implemented. This is a role that public health nurses can take in any setting; however, it occurs mainly in the public sector. Public health nurses contribute to systems for monitoring crucial health status indicators such as environmentally caused illnesses, immunization levels, infant mortality rates, and communicable disease occurrence in order to identify problems that threaten the public’s health and to develop effective interventions.
Examples of public health nursing activities include the following:
- Evaluating health trends and risk factors of population groups and helping to determine priorities for targeted interventions
- Working with communities or specific population groups within the community to develop public policy and targeted health promotion and disease prevention activities
- Participating in assessing and evaluating health care services to ensure that people are informed of available programs and services and assisted in the utilization of those services
- Providing essential input to interdisciplinary programs that monitor, anticipate, and respond to public health problems in population groups
- Providing health education, care management, and primary care to individuals and families who are members of vulnerable population and high-risk groups (Missouri Department of Health and Senior Services, n.d.)
Practice Application
➔ Setting the Scene
Legionnaires’ disease is a type of pneumonia caused by the Legionella bacteria. Legionella live in nature and rarely cause illness. In a human-made setting, Legionella can grow if water is not properly maintained. These water sources become a health problem when small droplets of water that contain the bacteria get into the air and people breathe them in (CDC, 2022b).
Legionnaires’ disease is a national reportable condition. In 2015, the New York City Department of Health and Mental Hygiene detected an abnormal number of Legionnaires’ cases in the South Bronx via the notifiable conditions public health policy. To better understand how and why this was occurring, the response team within the New York City Department of Health collected more data, including the sampling of many cooling towers within the designated areas within New York City in search of the Legionella pneumophilia bacteria. Using the surveillance data and information about the disease, such as where bacteria are commonly found, the health department was able to identify the source (environment) as a cooling tower in the city. Locating this tower led to the destruction of the bacteria within the tower and a new local law to test for Legionella as part of the required certification and maintenance inspections (Chamberlain et al., 2017).
➔ Think About It
- Imagine you were a nurse during this outbreak.
- How might you have been involved in the outbreak?
- What steps would you take if a patient came in with Legionnaires’ disease?
- Imagine you were a part of the New York City Department of Health and Mental Hygiene when this outbreak took place.
- What questions would you ask when the abnormal number of cases first arose?
- How would you answer these questions? What data would you need to collect?
- What were the three corners/vertices of the epidemiologic triangle for this outbreak?
- Why might Legionnaires’ disease be classified as a notifiable condition?
- Classify the following diseases as epidemic, pandemic, or cluster. Describe why you selected each classification.
- Malaria in Africa, which is present at all times because of the presence of infected mosquitoes
- Ebola in parts of Africa where it exceeds what is expected for this region
- COVID-19
- Measles, as occurring among employees at Factory X in Fort Worth, Texas, in March 1981
Additional Resources
CDC – Introduction to Epidemiology
CDC – Solve the Outbreak [Interactive Activity] (Accessible version and mobile app also available)
References
Arnold, S., Patterson, L., & Neill, C. (2020). Incidence vs prevalence and the epidemiologist’s bathtub. HSC Public Health Agency. www.publichealth.hscni.net/node/5277
Bovbjerg, M. L. (2020). Foundations of epidemiology. Oregon State University. https://open.oregonstate.education/epidemiology/
Centers for Disease Control and Prevention. (2014). Introduction to public health. www.cdc.gov/training/publichealth101/epidemiology.html
Centers for Disease Control and Prevention. (2015). Lesson 1: Understanding the epidemiologic triangle through infectious disease. www.cdc.gov/healthyschools/bam/teachers/documents/epi_1_triangle.pdf
Centers for Disease Control and Prevention. (2016). Teacher roadmap: Who are epidemiologists? https://web.archive.org/web/20230331203904/http://www.cdc.gov/careerpaths/k12teacherroadmap/epidemiologists.html
Centers for Disease Control and Prevention. (2018). Introduction to epidemiology. https://www.cdc.gov/training-publichealth101/php/training/introduction-to-epidemiology.html
Centers for Disease Control and Prevention. (2022a). Introduction to public health surveillance. https://www.cdc.gov/training/publichealth101/surveillance.html
Centers for Disease Control and Prevention. (2022b). About Legionnaires’ disease and Pontiac fever. https://www.cdc.gov/legionella/about/index.html
Chamberlain, A. T., Lehnert, J. D., & Berkelman, R. L. (2017). The 2015 New York City Legionnaires’ disease outbreak: A case study on a history-making outbreak. Journal of Public Health Management and Practice, 23(4), 410–416. https://doi.org/10.1097/PHH.0000000000000558
Missouri Department of Health and Senior Services. (n.d.). Role of public health nurses. Retrieved May 19, 2024, from https://health.mo.gov/living/lpha/phnursing/phnroles.php
Office of Disease Prevention and Health Promotion. (2022). Public health Infrastructure. Healthy People 2030. https://health.gov/healthypeople/objectives-and-data/browse-objectives/public-health-infrastructure