Epidemiology is as old as the discipline of medicine. The Greek physician Hippocrates wrote extensively around 400 B.C. about his medical observations. In one of Hippocrates’ most famous treatises, he discussed the importance of the physical, environmental, and social environment on the development of disease.
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 … 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.10
This line of thinking represented a rational account of the origins of disease and makes no mention of supernatural or religious causes of death or disease. While the scientific community conducted empirical investigation into the causes of disease, it was not common among the general public to make rational connections between the environment and health outcomes. As poverty has always been a risk factor for disease development, theologians, politicians, and even some physicians believed that diseases arose, at least in part, from consequences of laziness, weak constitutions, fear, impiousness, blasphemy, or other moral failings that were often imputed to people living in poverty.
During the early history of epidemiology, infectious diseases caused the largest burden of morbidity and mortality.11 Two thousand years after Hippocrates, in 1662, John Graunt published The Nature and Political Observations Made Upon the Bills of Mortality, which was the first epidemiologic study of weekly trends in natality and mortality among residents of London. This was one of the first applications of descriptive methods to compare population rates between important demographics such as age and sex and to quantify seasonal variation in disease occurrence. Graunt identified higher mortality rates among men compared to women and higher infant mortality compared to the general population. His work was an important foundational study not just for epidemiology but also the practical application of statistics to describe health issues.
Few epidemiology texts omit a discussion of John Snow’s investigation of cholera in London in the mid-1800s. At the time, the scientific community had no knowledge of germ theory, and the causes of the major infectious diseases were therefore unknown. Physicians and health officials believed the miasma theory, which followed that stagnant, foul-smelling air was the mechanism through which cholera and other mortal diseases infected humans. This line of thinking was common and had a long history. For example, in Hippocrates’ treatise On Airs, Waters and Places (quoted above), the weather and the air were the first major risk factors discussed. Physician John Snow hypothesized that, contrary to popular belief, infected water was the source of cholera outbreaks, though he did not have the technology to isolate the infectious agent, the bacterium Vibrio cholerae. Through keen observations and application of the scientific method, Dr. Snow identified that the proportion of deaths in the 1854 London cholera outbreak correlated with the source of people’s water. Specifically, neighborhood residents whose municipal water systems were sourced upstream of London had dramatically lower rates of cholera mortality compared to residents whose water system was from sources downstream, which included the unfiltered sewage of London residents. Based on the results of this natural experiment, and a door-to-door investigation of water-drinking behavior, illness, and death due to cholera in the Soho neighborhood of London, Dr. Snow convinced officials to remove the handle on the Broad Street water pump to remove the source of disease transmission. This series of epidemiologic investigations resulted in one of the first and most well documented evidence-based epidemiologic interventions. John Snow has since been called the father of modern epidemiology.
Because of improved living conditions, sanitation, and public health interventions such as vaccines, the major causes of morbidity and mortality in most of the world have transitioned from infectious in nature to non-infectious, or chronic disease. The Framingham Heart Study, which began in 1948, was an early and influential longitudinal study in which investigators assembled a cohort of almost 5,200 healthy, non-diseased residents of Framingham, Massachusetts, to evaluate the links between risk factors and chronic diseases. Seventy years later, this study continues and has provided the foundation for much of what we know about the links between lifestyle and morbidity and mortality due to cardiovascular disease.12 Since the 1950s and 1960s, researchers have established the causal link between smoking and lung cancer and diseases of almost all of the systems of the body. Smoking is, by far, the leading single cause of preventable death in the world.13 In the United States, smoking and secondhand smoke are responsible for almost 500,000 deaths per year.6