Introduction
Epidemiology is recognized as a core discipline within the field of public health. It is a unique discipline which formally began as a result of the sanitary reform movement in 17th and 18th century England. Epidemiology is formally defined in a number of ways. First, epidemiology is the study of the distribution and determinants of diseases and injuries in human populations (Mausner and Kramer, 1985). A second definition emphasizes the study of all factors, and their interdependence, that affect the occurrence of health and disease in populations. Finally, epidemiology is the study of the distribution and determinants of health‐related states and events in defined populations, and the application of this study to the control of health problems (Last, 1995).
Common to all of the preceding definitions is the concept of populations. Individuals are not the focus of epidemiology, but rather groups of individuals. Populations may represent large groups like the total population of the United States, as well as small groups such as the employees of a factory, store, or government agency. Central to the concept of populations is that groups of individuals exhibit certain commonalities. For example, a group of individuals who are related geographically, such as those living in the same city, represent a population. Also, a group of individuals who work in the same setting are a population. And a group of individuals who live and work together are a population, as in the case of military personnel. Groups of individuals of the same race or ethnic group are also considered populations.
Historically, epidemiology is a discipline that has experienced long and distinct development stages. It is reasonable to think that epidemiology began when man first walked on earth. The theory of “survival of the fittest” can be extended to assume that early man acquired, over time, an understanding of the relationship between environment and health. One simple example is the use of animal hides and furs as protective clothing.
The relationship between the environment and health and disease is mentioned in the Old Testament of the Bible. However, it wasn't until the Greek civilization was established that epidemiology began to emerge as a scientific discipline. Hippocrates, who lived from 460 to 377 B.C., wrote the classic work “On Airs, Waters, and Places.” His work began what is referred to today as environmental epidemiology. His writings discussed the link between the environment and human health. Hippocrates provided accurate descriptions of the diseases tetanus, typhus, and phthisis (Singer and Underwood, 1962). His contribution, which is the first documented use of observational techniques, earned Hippocrates the title of father of epidemiology and the designation as the first epidemiologist (Newcomb and Marshall, 1990).
Girolamo Fracastorius, who lived from 1478 to 1553, first proposed what is now known as germ theory (Ackerknecht, 1982). He studied epidemics and was the first to make a science‐backed statement of the nature of contagion, infection, disease germs, and modes of transmission. He identified ways in which infections can be transmitted. He discovered that infection was transmitted by direct contact, through droplet spread, from contaminated clothing, and through the air. Several hundred years later, Louis Pasteur would prove his theories were accurate.
Thomas Synenham, who lived in the 17th century, is called the English Hippocrates (Meynell, 1988). He re‐emphasized the theories of Hippocrates and expanded them to the 17th century. He was the first to describe the clinical manifestations of the condition known as Bell's Palsy. He re‐initiated scientific observations of health, Hippocrates' contribution, into the core fabric of modern epidemiology.
James Lind, who lived from 1716 to 1794, was the first known clinical epidemiologist. As a pioneer of naval hygiene, he worked as a surgeon's mate and sailed for many years around the world. He performed experiments in an attempt to determine the cause of scurvy. Scurvy, which causes loose teeth, bleeding gums, and hemorrhages, affected sailors. Lind adjusted their diets by adding foods such as cider, garlic, mustard, horseradish, vinegar, oranges, and lemons. He noted that the sailors who ate oranges and lemons recovered from the effects of scurvy, proving Lind's theory that citrus fruits were the best treatment for the disease. Today we know that scurvy is caused by a Vitamin C deficiency. Later in this life, Lind contributed to the knowledge of typhus fever on ships and chronicled diseases.
Medical registration of deaths began in Great Britain in 1801. William Farr (1807–1883), a statistical abstracter in the General Registry Office in London, established a national system of recording causes of death (Eyler, 1980). This standard classification system was the precursor to the International Classification of Diseases and Related Conditions (ICD), which will be discussed in Chapter Two. Farr's other contributions included involvement in the first modern census, use of the census to collect specific information on diseases and conditions (blindness and deafness), and invention of the standardized mortality rate (Newcomb and Marshall, 1990).
A colleague of William Farr, John Snow, used epidemiologic principles to study outbreaks of cholera in London in the 1850s (Lilienfeld, 2000). Snow demonstrated how scientific evidence can be used to support hypotheses and analytical investigations. He identified the source of the infectious agent, contaminated water in the Broad Street pump, and the etiology of the cholera outbreak (Collins, 2003). His work has been described as a brilliant use of descriptive and quantitative epidemiologic principles (Winkelstein, 1995).
As the 20th century began, epidemiology was involved with infectious and communicable diseases. The main cause of these diseases was overcrowded conditions in the cities of the world. In 1900, the leading causes of death were pneumonia and influenza, followed closely by tuberculosis. Other leading causes of death in 1900 were diarrhea, heart disease, and nephritis. As the 20th century progressed, chronic diseases became more pronounced as causes of death. In 1930, heart disease became the leading cause of death, as it is today. The emergence of chronic diseases as the leading health concern continued through the 1940s and 1950s, with infectious diseases becoming less of a concern. The difference between the death rates of chronic and infectious diseases was widening as the 20th century moved along. By the end of the 20th century, the only infectious diseases remaining in the top ten of leading causes of death were pneumonia and influenza.
The period of time demarcated by World War II is the beginning of another important period in the development of epidemiology as a scientific discipline. Epidemiology methods continued to evolve, with a focus on individual diseases and conditions. The case‐control study design was developed during the 1930s. Cohort studies were pursued to observe the relationship of tobacco usage and disease. Case‐control studies became very popular in hospital‐based studies, beginning in 1950 (Doll and Hill, 1950; Levin, Goldstein, and Gerhardt, 1950; Wynder and Graham, 1950). Since 1950 epidemiology has continued to develop, as cohort studies and clinical trials have gained popularity. Well‐known cohort studies include the Framingham Heart Study, the Bogalusa Heart Study (Gordon and others, 1977; Voors and others, 1976), and the Jackson Heart Study (Auerbach and others, 2017).
At the dawn of the 21st century, epidemiology has begun to expand its focus to health status, health‐related quality of life, and burden of disease. As a result of events of September 11, 2001, epidemiology has gained new roles in bioterrorism preparedness and management of health care services. With the significant number of emerging infectious diseases (including HIV/AIDS, SARS, and Zika), epidemiology's initial role in the study of epidemics will regain prominence.
Hospital care in the United States was formalized because of infectious disease. In 1798 President John Adams signed the Act for Relief of Sick and Disabled Sailors, which provide health care services to all naval personnel and established a network of marine hospitals in the country. This led to the opening of the first marine hospital in 1801, and subsequent establishment of the federal Marine Hospital Service, which was directed by the predecessor to today's Surgeon General. The importance of the Marine Hospital Service is that it began to require that physicians be adequately trained to provide health care services (U.S. Public Health Service, 2017).
The Marine Hospital Service was tasked through the National Quarantine Act of 1878 to prevent the introduction and spread of contagious and infectious disease, such as yellow fever and smallpox. Several years later the U.S. Congress expanded the responsibility of the Marine Hospital Service, which today is known as the U.S. Public Health Service, with the investigation of human disease (tuberculosis, hookworm, malaria, and leprosy), sanitation, water supplies, and sewerage (U.S. Public Health Service, 2017).