Chapter Summary

Epidemiology is the scientific study of the distribution and determinants of diseases and health outcomes to improve the health and safety of populations by establishing causal factors for health issues. Epidemiological objectives include identifying the cause of disease or factors that increase an individual’s risk for disease, identifying how diseases are transmitted, determining the extent of disease in a community, evaluating preventive and therapeutic measures of health care delivery, and providing the foundation for developing public policy regarding disease prevention and health promotion. By identifying causes and risk factors of disease development, public health nurses can intervene early to reduce morbidity and mortality from the disease. Epidemiology provides the opportunity to develop an evidence-based foundation for prevention programs.

Epidemiology has been traced back to Greek physician Hippocrates circa 400 BCE. Since then, epidemiologists Edward Jenner, Ignaz Semmelweis, John Snow, and Florence Nightingale, among others, helped to shape epidemiology into the science and the action of public health. They used their observation skills and disseminated their findings widely to gain support from politicians and other government officials and to promote the health and well-being of the public.

The epidemiologic triad describes disease as a result of the relationship among a susceptible host, an external agent, and an environment that brings the host and agent together. The chain of infection is an epidemiological model that depicts the complex, nuanced interplay between the host, agent, and environment and includes a reservoir, portal of exit, mode of transmission, agent, portal of entry, and host. The natural history of disease follows diseases through stages: susceptibility in a host, subclinical disease where disease carriers become a concern, and clinical disease in which overt symptoms are present. Understanding these epidemiological approaches enables nurses to direct the most appropriate public health intervention at any link in the chain of infection.

Descriptive epidemiology covers the time, place, and person of epidemiologic events and seeks to describe disease variables. Analytic epidemiology searches for the why and how of diseases or other public health issues by testing hypotheses about causal relationships. In experimental studies, the investigator determines the exposure for the participants. In observational studies, participants are exposed under natural conditions. Cohort studies, case-control studies, and cross-sectional studies are the most common types of observational studies used in epidemiological research.

Ratios can be used as a descriptive measure or to calculate the occurrence of illness or death between two groups. Proportions are often used to describe the extent of disease attributable to a particular exposure. A rate measures the frequency of an event’s occurrence in a specified population over a defined period. Epidemiologists use rates to describe incidence, prevalence, case-fatality, and attack rates. Measures of morbidity characterize how many individuals in a population become or are ill at a specified time. Mortality rate measures the frequency of death in a defined population during a specific interval. Measures of association assess the degree to which the risk of disease increases when exposed to an agent, thereby demonstrating the strength of a causal relationship.

Research translation applies scientific research and evidence to a practical setting. Population and public health nurses can translate epidemiological research into a tool to help prevent or address health events and diseases. Dissemination is the distribution of information to a particular audience to spread knowledge about a disease and its associated interventions.

Epidemiologic research can help identify problems in a community or population, assist in discovering the etiology of a problem or risk factors for its development, and guide nurses and other public health professionals toward evidence-based interventions and recommendations to address the issue. When rigorously conducted, reported, and communicated, epidemiological studies can result in major health policy changes. Findings from epidemiological studies affect population health from a primary and secondary prevention lens and as a way to control disease.