The Greek physician
Hippocrates, taught by Democritus, was known as the father of
medicine, sought a logic to sickness; he is the first person known to have examined the relationships between the occurrence of disease and environmental influences. Hippocrates believed sickness of the human body to be caused by an imbalance of the four
humors (black bile, yellow bile, blood, and phlegm). The cure to the sickness was to remove or add the humor in question to balance the body. This belief led to the application of bloodletting and dieting in medicine. He coined the terms
endemic (for diseases usually found in some places but not in others) and
epidemic (for diseases that are seen at some times but not others).
Modern era In the middle of the 16th century, a doctor from
Verona named
Girolamo Fracastoro was the first to propose a theory that the very small, unseeable, particles that cause disease were alive. They were considered to be able to spread by air, multiply by themselves and to be destroyable by fire. In this way he refuted
Galen's
miasma theory (poison gas in sick people). In 1543 he wrote a book
De contagione et contagiosis morbis, in which he was the first to promote personal and environmental
hygiene to prevent disease. The development of a sufficiently powerful microscope by
Antonie van Leeuwenhoek in 1675 provided visual evidence of living particles consistent with a
germ theory of disease. During the
Ming dynasty,
Wu Youke (1582–1652) developed the idea that some diseases were caused by transmissible agents, which he called
Li Qi (戾气 or pestilential factors) when he observed various epidemics rage around him between 1641 and 1644. His book
Wen Yi Lun (瘟疫论, Treatise on Pestilence/Treatise of Epidemic Diseases) can be regarded as the main etiological work that brought forward the concept. His concepts were still being considered in analysing SARS outbreak by WHO in 2004 in the context of traditional Chinese medicine. Another pioneer,
Thomas Sydenham (1624–1689), was the first to distinguish the fevers of Londoners in the later 1600s. His theories on cures of fevers met with much resistance from traditional physicians at the time. He was not able to find the initial cause of the
smallpox fever he researched and treated. showing the
clusters of cholera cases in the
London epidemic of 1854 John Snow is famous for his investigations into the causes of the 19th-century
cholera epidemics, and is also known as the father of (modern) Epidemiology. He began with noticing the significantly higher death rates in two areas supplied by Southwark Company. His identification of the
Broad Street pump as the cause of the Soho epidemic is considered the classic example of epidemiology. Snow used chlorine in an attempt to clean the water and removed the handle; this ended the outbreak. This has been perceived as a major event in the history of
public health and regarded as the founding event of the science of epidemiology, having helped shape public health policies around the world. However, Snow's research and preventive measures to avoid further outbreaks were not fully accepted or put into practice until after his death due to the prevailing
Miasma Theory of the time, a model of disease in which poor air quality was blamed for illness. This was used to rationalize high rates of infection in impoverished areas instead of addressing the underlying issues of poor nutrition and sanitation, and was proven false by his work. Other pioneers include Danish physician
Peter Anton Schleisner, who in 1849 related his work on the prevention of the epidemic of
neonatal tetanus on the
Vestmanna Islands in
Iceland. Another important pioneer was
Hungarian physician
Ignaz Semmelweis, who in 1847 brought down infant mortality at a Vienna hospital by instituting a disinfection procedure. His findings were published in 1850, but his work was ill-received by his colleagues, who discontinued the procedure. Disinfection did not become widely practiced until British surgeon
Joseph Lister, aided by his college, chemist
Thomas Anderson, was able to "discover"
antiseptics in 1865 based on the earlier work of
Louis Pasteur. In the early 20th century, mathematical methods were introduced into epidemiology by
Ronald Ross,
Janet Lane-Claypon,
Anderson Gray McKendrick, and others. In 1927,
William Ogilvy Kermack and
Anderson Gray McKendrick published the foundational
SIR (Susceptible-Infected-Recovered) model, applying
mass-action kinetics from
chemistry to
disease transmission in populations. In a parallel development during the 1920s, German-Swiss pathologist
Max Askanazy and others founded the International Society for Geographical Pathology to systematically investigate the geographical pathology of cancer and other non-infectious diseases across populations in different regions. After World War II,
Richard Doll and other non-pathologists joined the field and advanced methods to study cancer, a disease with patterns and mode of occurrences that could not be suitably studied with the methods developed for epidemics of infectious diseases. Geography pathology eventually combined with infectious disease epidemiology to make the field that is epidemiology today. Another breakthrough was the 1954 publication of the results of a
British Doctors Study, led by
Richard Doll and
Austin Bradford Hill, which lent very strong statistical support to the link between
tobacco smoking and
lung cancer. In the late 20th century, with the advancement of biomedical sciences, a number of molecular markers in blood, other biospecimens and environment were identified as predictors of development or risk of a certain disease. Epidemiology research to examine the relationship between these
biomarkers analyzed at the molecular level and disease was broadly named "
molecular epidemiology". Specifically, "
genetic epidemiology" has been used for epidemiology of germline genetic variation and disease. Genetic variation is typically determined using DNA from peripheral blood leukocytes.
21st century Since the 2000s,
genome-wide association studies (GWAS) have been commonly performed to identify genetic risk factors for many diseases and health conditions. While most molecular epidemiology studies are still using conventional disease
diagnosis and classification systems, it is increasingly recognized that disease progression represents inherently heterogeneous processes differing from person to person. Conceptually, each individual has a unique disease process different from any other individual ("the unique disease principle"), considering uniqueness of the
exposome (a totality of endogenous and exogenous / environmental exposures) and its unique influence on molecular pathologic process in each individual. Studies to examine the relationship between an exposure and molecular pathologic signature of disease (particularly
cancer) became increasingly common throughout the 2000s. However, the use of
molecular pathology in epidemiology posed unique challenges, including lack of research guidelines and standardized
statistical methodologies, and paucity of interdisciplinary experts and training programs. Furthermore, the concept of disease heterogeneity appears to conflict with the long-standing premise in epidemiology that individuals with the same disease name have similar etiologies and disease processes. To resolve these issues and advance population health science in the era of molecular
precision medicine, "molecular pathology" and "epidemiology" was integrated to create a new interdisciplinary field of "
molecular pathological epidemiology" (MPE), defined as "epidemiology of molecular pathology and heterogeneity of disease". In MPE, investigators analyze the relationships between (A) environmental, dietary, lifestyle and genetic factors; (B) alterations in cellular or extracellular molecules; and (C) evolution and progression of disease. A better understanding of heterogeneity of disease
pathogenesis will further contribute to elucidate
etiologies of disease. The MPE approach can be applied to not only neoplastic diseases but also non-neoplastic diseases. The concept and paradigm of MPE have become widespread in the 2010s. By 2012, it was recognized that many pathogens'
evolution is rapid enough to be highly relevant to epidemiology, and that therefore much could be gained from an interdisciplinary approach to infectious disease integrating epidemiology and
molecular evolution to "inform control strategies, or even patient treatment." Modern epidemiological studies can use advanced statistics and
machine learning to create
predictive models as well as to define treatment effects. There is increasing recognition that a wide range of modern data sources, many not originating from healthcare or epidemiology, can be used for epidemiological study. Such digital epidemiology can include data from internet searching, mobile phone records and retail sales of drugs. == Types of studies ==