Chapter Four: Epidemiology

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.

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

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).

Surveillance and the Epidemiologic Triangle

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.