1.1 Epidemiology and its uses
What is epidemiology?
The word epidemiology is based on the Greek roots epi (upon), demos (the people, as in “democracy” and “demography”), and logia (“speaking of,” “the study of”). Specific use of the term in the English language dates to the mid-19th century (Oxford English Dictionary), around the time the London Epidemiological Society was founded in 1850. Since then, epidemiology has defined itself in many ways, including:
- the study of the distribution and determinants of diseases and injuries in populations (Mausner and Baum, 1974);
- the study of the occurrence of illness (Gaylord Anderson cited in Cole, 1979, p. 15);
- a method of reasoning about disease that deals with biological inferences derived from observations of disease phenomena in population groups (Lilienfeld, 1978b, p. 89);
- the quantitative analysis of the circumstances under which disease processes, including trauma, occur in population groups, and factors affecting their incidence, distribution, and host responses, and the use of this knowledge in prevention and control (Evans, 1979, p. 381).
A widely accepted contemporary definition of epidemiology identifies the discipline as “the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to control of health problems” (Last, 2001).
The word epidemiology is, of course, based on the word epidemic. This term dates back to the time of Hippocrates, circa 400 BCE. Until not too long ago, epidemic referred only to the rapid and extensive spread of an infectious disease within a population. Now, however, the term applies to any health-related condition that occurs in clear excess of normal expectancy. For example, one may hear mention of an “epidemic of teen pregnancy” or an “epidemic of violence.” This broader use of the term reflects epidemiology’s expansion into areas beyond infectious disease control to include the study of health and health-related determinants in general. In this non-limiting sense, epidemiology is still the study of epidemics and their prevention (Kuller, 1991).
In addition, epidemiology is becoming increasingly integrated in biomedical research and health care. Note, however, that the main distinction between epidemiology and clinical medicine is their primary unit of concern. The primary unit of concern for the epidemiologist is “an aggregate of human beings” (Greenwood, 1935). Compare this with clinical medicine, whose main unit of concern is the individual. A metaphor that compares epidemiology with clinical medicine discusses a torrential storm that causes a break in the levees. People are being washed away in record numbers. Under such circumstances, the physician’s task is to offer lifejackets to people one at a time. In contrast, the epidemiologist’s task is to stem the tide of the flood to mitigate the problem and prevent future occurrences.
What is public health?
Like epidemiology, public health has been defined in many different ways including “organized community effort to prevent disease and promote health (Institute of Medicine, 1988) and “one of the efforts organized by society to protect, promote, and restore the people’s health (Last 2001). By any definition, the aim of public health is to reduce injury, disability, disease, and premature death in the population. Public health is thus a mission comprising many activities, including but not limited to epidemiology. Epidemiology is a “study of” with many applications, while public health is an undertaking.
Note that epidemiology is one of the core disciplines of public health. Other core disciplines in public health include biostatistics, environmental health sciences, health policy and management, and social and behavioral sciences (Calhoun et al., 2008). The practice of public health also requires cross-cutting interdisciplinary competencies in areas such as communication, informatics, culture and diversity, and public health biology.
What is health?
Health itself is not easily defined. The standard medical definition of health is “the absence of disease.” Dis-ease, literally the absence of “ease,” is when something is wrong with a bodily or mental function. The World Health Organization in the preamble to its 1948 constitution defined health as “a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity.”
Walt Whitman (1954, p. 513), in his poetic way, defined health as:
the condition [in which] the whole body is elevated to a state by other unknown—inwardly and outwardly illuminated, purified, made solid, strong, yet buoyant. A singular charm, more than beauty, flickers out of, and over, the face—a curious transparency beams in the eyes, both in the iris and the white—temper partakes also. The play of the body in motion takes a previously unknown grace. Merely to move is then a happiness, a pleasure—to breathe, to see, is also. All the before hand gratifications, drink, spirits, coffee, grease, stimulants, mixtures, late hours, luxuries, deeds of the night seem as vexatious dreams, and now the awakening; many fall into their natural places, wholesome, conveying diviner joys.
This passage from Whitman address quality of life, an area of increasing interest to epidemiologists.
Additional useful terms
One of the ten American Schools of Public Health MPH Epidemiology competencies is to “apply the basic terminology and definitions of epidemiology” (Calhoun et al., 2008). Therefore, terminology will be introduced throughout this book. Table 1.1 lists definitions for several standard terms. For example, an epidemic is the occurrences of disease in clear excess of normalcy, while a pandemic is an epidemic that affects several countries or continents. An endemic disease is one that is consistently present in the environment. The term endemic is also used to refer to a normal or usual rate of disease. An excellent source for epidemiologic definitions is The Dictionary of Epidemiology (Porta, 2008), which is updated periodically.
Table 1.1 Selected terms briefly defined.
| Epidemiology: the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems |
| Public health: organized effort to prevent disease and promote health |
| Endemic: occurring at a consistent or regular rate |
| Epidemic: occurring in clear excess of normalcy |
| Pandemic: an epidemic that affects several countries or continents |
| Morbidity: related to or caused by disease or disability |
| Mortality: related to death |
Some terms used in the field are not readily defined in a singular way. For example, some sources differentiate between disease, illness, and sickness. Susser (1973) defines disease as the medically applied term for a physiological or psychological dysfunction; illness is what the patient experiences; and sickness is the state of dysfunction of the social role of an ill person. In contrast, one source considers “disease” a subtype of “illness” (Miettinen and Flegel, 2003). While yet in other contexts, “disease” is merely a general term used to refer to any health-related outcome or condition. Thus, the use of epidemiologic terminology is context specific and is, at times, controversial.
Uses of epidemiology
Epidemiologic practice is characterized by a close connection between the scientific study of the causes of disease, and the application of this knowledge to treatment and prevention (especially the later). The discipline covers a broad range of activities, including conducting biomedical research, communicating research findings, and participating with other disciplines and sectors in deciding on public health practices and interventions.
A sample of epidemiology’s varied concerns include studies of the effects of environmental and industrial hazards, studies of the safety and efficacy of medicines and medical procedures, studies of maternal and child health, studies of food safety and nutrition, studies of the long-term effects of diet and lifestyle, surveillance and control of communicable and noncommunicable diseases, ascertainment of personal and social determinants of health and ill-health, medico-legal attribution of risk and responsibility, screening and early detection of the population for disease, and the study of health-care services. Because findings from epidemiologic investigations are linked to health policy, epidemiologic studies often have important legal, financial, and political consequences.
More than half a century ago, Morris (1957) described seven uses of epidemiology. These seven uses, listed in Table 1.2, have stood the test of time. The seventh use, search for causes, is perhaps the most important current application because of its essential role in effective disease prevention.
Table 1.2 General uses of epidemiology (Morris, 1957).
1 In historical study of the health of the community and of the rise and fall of diseases in the population; useful “projections” into the future may also be possible. 2 For community diagnosis of the presence, nature, and distribution of health and disease among the population, and the dimensions of these in incidence, prevalence, and mortality; taking into account that society and health problems are changing. 3 To study the workings of health services. This begins with the determination of needs and resources, proceeds to analysis of services in action and, finally, attempts to appraise. Such studies can be comparative between various populations. 4 To estimate, from the common experience, the individual’s chances and risks of disease. 5 To help complete the clinical picture: by including all types of cases in proportion; by relating clinical disease to subclinical; by observing secular changes in the character of disease, and its picture in other countries. 6 In identifying syndromes from the distribution of clinical phenomena among sections of the population. 7 In the search for causes of health and disease, starting with the discovery of groups with high and low rates, studying these differences in relation to differences in ways of living; and, where possible, testing these notions in actual practice among populations. |
1.2 Evolving patterns of morbidity and mortality
Twentieth century changes in demographics and disease patterns
The theory of epidemiologic transition focuses on the dramatic changes in morbidity and mortality that have occurred in relation to demographic, biologic, and socioeconomic factors during the 20th century (Omran, 1971). Ample evidence exists to document a transition from infectious diseases as the predominant causes of morbidity and mortality to a predominance of noninfectious diseases (Table 1.3). The transition from predominantly infectious to noninfectious causes resulted from changes in society at large and improvements in medical technology. Steady economic development led to better living conditions, improved nutrition, decreases in childhood mortality, diminished fertility rates, and technological advances in medicine.
Table 1.3 Leading causes of death in the United States, 1900 and 2007a.
| Rank | 1900b | 2007c |
| 1. | Pneumonia (all forms) and influenza [202.2] | Diseases of the heart [204.3] |
| 2. | Tuberculosis (all forms) [194.4] | Malignant neoplasms (cancers) [186.6] |
| 3. | Diarrhea, enteritis, and ulceration of the intestines [142.7] | Cerebrovascular diseases (stroke) [45.1] |
| 4. | Diseases of the heart [137.4] | Chronic lower respiratory diseases [42.4] |
| 5. | Intracranial lesions of vascular origin [106.9] | Accidents (unintentional injuries) [41.0] |
| 6. | Nephritis (all forms) [88.6] | Alzheimer’s disease [24.7] |
| 7. | All accidents [72.3] | Diabetes mellitus (diabetes) [23.7] |
| 8. | Cancer and other malignant tumors [64.0] | Influenza and pneumonia [17.5] |
| 9. | Senility [50.2] | Nephritis, nephrotic syndrome and nephrosis (kidney diseases) [15.4] |
| 10. | Diphtheria [40.3] | Septicemia [11.5] |
aCrude death rates per 100 000 are listed in square brackets. Rates have not been adjusted for age differences in the population and, therefore, should not be compared between time periods.
bSource: National Office of Vital Statistics, 1947.
cSource: Xu et al., 2010.
Decreases in mortality and fertility led to a substantial shift in the age distribution of populations, especially in industrialized societies, a phenomenon known as the demographic transition (Figure 1.1). With this now familiar demographic shift came a concomitant rise in age-related diseases such as atherosclerotic cardiovascular and cerebrovascular disease, cancer, chronic lung disease, diabetes and other metabolic diseases, liver disease, musculoskeletal disorders, and neurological disorders. Many of these noncontagious diseases are thought to have important lifestyle components rooted in behaviors such as smoking, dietary excesses, and physical inactivity (“diseases of civilization”). As of the mid-20th century, these prevalent chronic diseases were viewed primarily as an intrinsic property of aging (so-called degenerative diseases). Now, however, these diseases are regarded as a diverse group of pathologies with varied and complex etiologies. What brings them together as a group is their insidious onset, long duration, and the fact that they seldom resolve spontaneously.
Figure 1.1 Population pyramids for the United States, 1900, 1950, and 2000.
(Sources: Bureau of the Census, 1904; U.S. Census Bureau International Data Base, 2002).
By the middle of the 20th century, epidemiologists came to realize that the limited tools they had developed to address acute infectious diseases were no longer sufficient in studying chronic ailments. Out of this awareness arose development of new investigatory tools—field surveys, cohort studies, case–control studies, and clinical trial—as will be addressed later in this book. Using these newly developed methods, epidemiologists identified risk factors that influence the incidence of many chronic conditions (Table 1.4).
Table 1.4 Chronic diseases and their relation to selected, modifiable risk factors: + = established risk factor and ± = possible risk factor.
Mortality trends since 1950
Figure 1.2 displays age-adjusted mortality rates for all causes combined and the six leading causes of death in the United States in 2006 for the years 1950 through 2006. Rates are plotted on a logarithmic scale, so even modest downward slopes represent large changes in the rates of occurrence. During this period, age-adjusted mortality for all causes combined decreased from 1446.0 per 100 000 in 1950 to 776.5 per 100 000, a 47% decline. An important component of this decline came from advances in preventing cardiovascular and cerebrovascular mortality. In 1950, mortality from heart disease occurred at the adjusted rate of 588.8 per 100 000. By 1992, this rate was cut by two-thirds, to 200.2 per 100 000.
Figure 1.2 Age-adjusted death rates from 1950 to 2006, the United States, for the six leading causes of death in 2006.
(Source: CDC/NCHS 2010).
Trends in life expectancy
Life expectancy is the average number of years of life a person is expected to live if current mortality rates in the population were to remain constant. In 1900, life expectancy at birth in the United States was 47.3 years. By 2006, life expectancy was 77.7 years (75.1 years for men and 80.2 years for women). Figure 1.3 charts this dramatic progress.
Figure 1.3 Life expectancy at birth and at age 65 by sex, United States, 1900–2003.
(Source: CDC/NCHS, 2006).
During the early part of the 20th century, increases in life expectancy can be traced to decreases in mortality at younger ages due primarily to improved sanitization and hygiene, improved nutrition, smaller family size, better provision of uncontaminated water, control of infectious disease vectors, pasteurization of milk, better infant and child care, and immunization (Doll, 1992). Since the middle of the century 20th century, life expectancy at older ages has shown significant increases. In 1950, a 65 year old man had a life expectancy of 12.8 remaining years; by 2000 this has increased to 16.0 years; by 2006 this had increased to 17.0 years (CDC/NCHS, 2010). For women, comparable increases have occurred. These increases can be traced to technological improvements in medical care (e.g., antibiotics, improvements in the safety of surgery, treatment of hypertension, etc.), dietary changes, avoidance of smoking, reductions in vascular diseases, and the pharmacologic control of high blood pressure and hyperlipidemia (Doll, 1992).
1.3 Selected historical figures and events
A knowledge of epidemiological history, combined with a firm grasp of the statistical method were as essential parts of the outfit of the investigator in the field as was a grounding in bacteriology.
Major Greenwood
Roots of epidemiology
Epidemiological insights into health and disease are probably as old as civilization itself. The Old Testament refers to the benefits of certain diets, the Greeks linked febrile illnesses to environmental conditions (“marsh fever”), and the Romans recognized the toxic effects of consuming wine from lead-glazed pottery.
Hippocrates (circa 460–388 BCE) is said to have prepared the groundwork for the scientific study of disease by freeing the practice of medicine from the constraints of philosophical speculation, superstition, and religion, while stressing the importance of careful observation in identifying natural factors that influenced health. In Air, Waters, and Places (Table 1.5), Hippocrates refers to environmental, dietary, behavioral, and constitutional determinants of disease. “From these things, we must proceed to investigate everything else.” Elsewhere, Hippocrates provides accurate descriptions of various clinical ailments, including tetanus, typhus, and tuberculosis.
Table 1.5 Part I of On Air, Waters, and Places (Hippocrates, 400 BCE).
| Whoever wishes to investigate medicine properly, should proceed thus: in the first place to consider the seasons of the year, and what effects each of them produces for they are not at all alike, but differ much from themselves in regard to their changes. Then the winds, the hot and the cold, especially such as are common to all countries, and then such as are peculiar to each locality. We must also consider the qualities of the waters, for as they differ from one another in taste and weight, so also do they differ much in their qualities. In the same manner, when one comes into a city to which he is a stranger, he ought to consider its situation, how it lies as to the winds and the rising of the sun; for its influence is not the same whether it lies to the north or the south, to the rising or to the setting sun. These things one ought to consider most attentively, and concerning the waters which the inhabitants use, whether they be marshy and soft, or hard, and running from elevated and rocky situations, and then if saltish and unfit for cooking; and the ground, whether it be naked and deficient in water, or wooded and well watered, and whether it lies in a hollow, confined situation, or is elevated and cold; and the mode in which the inhabitants live, and what are their pursuits, whether they are fond of drinking and eating to excess, and given to indolence, or are fond of exercise and labor, and not given to excess in eating and drinking. |
A long period of relative quiescence in scientific medicine followed the Hippocratic era. In the 17th century scientific observation in medicine began to reawaken, dawning an upcoming Age of Enlightenment in the 18th century. This period is credited with the development of scientific methods based on systematized observation, experimentation, measurement, and a multistep process that advanced from theory to conclusion by testing and revising causal hypotheses. In summarizing the profound impact brought about by these changes, Ariel and Will Durant (1961, p. 601) wrote:
Science now began to liberate itself from the placenta of its mother, philosophy. It developed its own distinctive methods, and looked to improve the life of man on the earth. This movement belonged to the heart of the Age of Reason, but it did not put its faith in “pure reason”—reason independent of experience and experiment. Reason, as well as tradition and authority was now to be checked by the study and record of lowly facts; and whatever logic might say, science would aspire to accept only what could be quantitatively measured, mathematically expressed, and experimentally proved.
The features of scientific work—measuring, sequencing, classifying, grouping, confirming, observing, formulating, questioning, identifying, generalizing, experimenting, modeling, and testing—now took prominence.
A very early reawakening came with the work of the “English Hippocrates” Thomas Sydenham (1624–1689). Like Hippocrates, Sydenham stressed the need for careful observation for the advancement of health care. Using information combed from patients’ records, Sydenham wrote about the prevalent diseases of his day. In a similar vein, Sydenham’s contemporary Bernardino Ramazzini (1633–1714) published his comprehensive work The Diseases of Workers (De Morbis Artificum Diatriba). The Diseases of Workers discussed the hazards of various environmental irritants (chemicals, dust, metals, and abrasive agents) encountered in 52 different occupations. Renowned as an early expositor of specificity in linking environment cause to disease, Ramazzini set the stage for occupational medicine and environmental epidemiology. Not long after Ramazzini, the Englishman Percival Pott (1713–1788) identified chimney soot as the cause of enormously elevated rates of scrotal cancer in chimney sweeps (Pott, 1775/1790). This may have been the first link demonstrating a causal association between a malignancy and an environmental carcinogen.
John Graunt
The development of systems to collect the causes of death on a population basis was key to the development of epidemiology. The earliest tallying of deaths dates back to the reign of the Black Death (bubonic plague), when in the 14th and 15th centuries officials in Florence and Venice began keeping records of the number of persons dying, specifying cause of death in broad terms, such as plague/not plague (Saracci, 2001).
In England, the collection of death certificates began in selected parishes in 1592. However, it was not until the middle of the 17th century that this resource started to be used in an epidemiologic way by an intellectually curious London haberdasher by the name of John Graunt (1620–1674; Figure 1.4). Graunt tallied mortality statistics and made many forward-looking and insightful interpretations based these tallies in his publication Natural And Political Observations Mentioned In A Following Index And Made Upon The Bills Of Mortality (1662). Among his many observations, Graunt noted regional differences in mortality, high mortality in children (one-third of the population died before the age of 5), and greater mortality in men than women despite higher rates of physician visits in women (a phenomenon that still exists today). He noted that more boys than girls were born, debunked inflated estimates of London’s population size, noted that population growth in London was due mostly to immigration, determined that plague claimed more deaths than originally thought, and documented an epidemic of rickets.
By starting with a hypothetical group of 100 people, Graunt constructed one of the first known life tables as follows. Out of 100 people born, Graunt projected the following expectations for survival (O’Donnell, 1936):
| At the end of 6 years | 64 of the initial 100 would be alive |
| At the end of 16 years | 40 of the initial 100 would be alive |
| At the end of 26 years | 25 of the initial 100 would be alive |
| At the end of 36 years | 16 of the initial 100 would be alive |
| At the end of 46 years | 10 of the initial 100 would be alive |
| At the end of 56 years | 6 of the initial 100 would be alive |
| At the end of 60 years | 3 of the initial 100 would be alive |
| At the end of 76 years | 1 of the initial 100 would be alive |
| At the end of 80 years | 0 of the initial 100 would be alive |
Graunt recognized the importance of systematized record collection, was fastidious in his concern for accuracy, and took great care in scrutinizing the origins of data while being aware that certain forms of death tended to be misclassified. Given the period in which he lived and the limitations of its data, these are remarkable insights. It is therefore not surprising that many modern epidemiologists trace the birth of their discipline to Graunt’s remarkable work. Rothman (1996) proffers the following lessons modern epidemiologists can learn from Graunt:
- He was brief.
- He made his reasoning clear.
- He subjected his theories to repeated and varied tests.
- He invited criticism of his work.
- He was willing to revise his ideas when faced with contradictory evidence.
- He avoided mechanical interpretations of data.
Despite his brilliance with numbers, John Graunt was not a good money manager. He died bankrupt on Easter-eve 1674 and was buried under what was then a pigsty in St. Dunstan’s Church in Fleet Street. His eulogy read, “what pitty ’tis so great an ornament of the city should be buryed so obscurely!” (Aubrey, 1949).
Germ theory
The notion of a living agent as a cause of disease had been around since ancient times. For instance, the Roman poet Lucretius (circa 100 BC) refers to the seeds of disease passing from healthy to sick individuals in the poem De Rerum Natura. However, the first cogent germ theory was presented by Girolamo Fracastoro in 1546 (Saracci, 2001).
Despite early theories of contagion, the prevailing theories of epidemics in the 19th century were expressed in terms of “spontaneous generation” and “miasma atmospheres.” This manner of thinking began to change midcentury when in 1840 Jakob Henle (1809–1885) presented his treatise of the contagium animatum in which he theorized that a living substance multiplied within the body where it was excreted by sick individuals and communicated to healthy individuals.
During the same era, John Snow (1813–1858) was independently developing similar ideas about contagion, basing his theories on the epidemiologic and pathophysiologic features of cholera. Among Snow’s early epidemiologic observations was how cholera spread along the routes of human commerce and war and was propagated from human to human. Among his pathophysiologic observations was the cholera was primarily a gastrointestinal disease and that the loss of fluids caused its systemic effect by means of “internal congestion” (sludging of the blood and hypovolemic shock). Snow’s theory of contagion recognized that infection with a stabile living organism was necessary for transmission to occur and that the infectious agent multiplies after infections to produce its effects (Winkelstein, 1995). Later in this chapter we will discuss three of Snow’s seminal epidemiologic studies.
The French chemist Louis Pasteur (1822–1895) ultimately put the doctrine of spontaneous generation to rest by demonstrating that fermentation and organic decay were produced by microorganisms. Pasteur was also the first to isolate an agent responsible for an epidemic disease (in silk worms, in 1865), found that septicemia was caused by anaerobic bacterium, and developed the process for killing germs by heating that still bears his name (“pasteurization”).
Henle’s student Robert Koch (1843–1910) made a breakthrough when he decided to stain microbes with dye, enabling him to visualize the microbe that caused tuberculosis in 1882 and the cholera bacillus in 1883. Koch is also known for his Postulates, which he developed in 1890.
Until the discovery of arthropod (insect borne) transmission of Texas cattle fever, the only known modes of transmission for infectious agents were by water and air. In 1882, Daniel E. Salmon (1850–1914) realized that Texas cattle fever presented something unusual—the disease stayed below a geographic line that extended through the southern United States and Mexico (Figure 1.5) and was not conveyed from bovine to bovine directly or through the atmosphere. Using various epidemiologic and laboratory methods, he and a team of workers at the U.S. Department of Agriculture conducted a series of experiments that demonstrated the vector-borne transmission of the disease. This was the first demonstration of a complex web of causation involving an agent (Babesia bigeminal) being transmitted to a mammalian host (cattle) through an invertebrate vector (the tick Boophilus angulatus). Discoveries of invertebrate vectors for other diseases (e.g. malaria, yellow fever) soon followed. The complex interactions involved in the maintenance and transmission of an agent in the environment provided the first theories of medical ecology.
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