Epidemiology is a discipline which has evolved with the changes taking place in society and the emergence of new diseases and new discipline related to epidemiology. With these evolutions, it is important to understand epidemiology and to analyse the evolution of content of definitions of epidemiology.
The main objective of this paper was to identify new definitions of epidemiology available since 1978. Secondary objectives were to analyse the content of these definitions, to compare them with those used by Lilienfeld and to determine whether changes have taken place over the last forty years.
A review of grey literature and published literature was conducted to find the definitions of epidemiology written between 1978 and 2017.
102 definitions of epidemiology were retained. They helped to highlight 20 terms and concepts related to epidemiology. Most of them were already used in the definitions used by Lilienfeld. Five terms were present in more than 50% of definitions from the period 1978 to 2017: “population”, “study”, “disease”, “health” and “distribution”. Several developments have occurred: strengthening of the terms “control” and “health” already used, the concept of “disease” was less frequently encountered whereas the concepts “infectious diseases”, “mass phenomenon” are no longer used in definitions from 1978 to 2017.
Citation: Frérot M, Lefebvre A, Aho S, Callier P, Astruc K, Aho Glélé LS (2018) What is epidemiology? Changing definitions of epidemiology 1978-2017. PLoS ONE 13(12): e0208442. https://doi.org/10.1371/journal.pone.0208442
Editor: Paul H. Mason, Monash University, AUSTRALIA
Received: June 12, 2018; Accepted: November 12, 2018; Published: December 10, 2018
Copyright: © 2018 Frérot et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the paper.
Funding: The authors received no specific funding for this work.
Competing interests: The authors have declared that no competing interests exist.
Epidemiology is a recent discipline which has evolved with the changes taking place in society and the emergence of new diseases. This evolution has allowed epidemiology to remain a useful and relevant tool in bringing to light and understanding diseases and health events. Since its origins, more than a century ago, many definitions of epidemiology have been suggested.
In 1978, Lilienfeld published a seminal paper on definitions of epidemiology. For him, there was no consensus among epidemiologists as to the definition of epidemiology. The objective of his work was to provide a single and understandable definition of epidemiology that was suitable for all types of diseases and populations. Lilienfeld based his work on 23 existing definitions of epidemiology and proposed the following definition : "Epidemiology is a method of reasoning about disease that deals with biological inference derived from observations of disease phenomena in population groups".
The publication of this article resulted in many comments, including three letters to the Editor  each discussing the limits of this new definition. In his letter, Evans analyzed the content of the 23 definitions used reviewed by Lilienfeld. He listed the various terms and concepts used in these definitions and determined their frequency of use. After this work, he proposed a different definition of epidemiology : "Epidemiology is the quantitative analysis of the circumstances under which disease processes, including trauma, occur in population groups, factors affecting their incidence, distribution, and the host response and use of this knowledge in prevention and control ".
Since the work of Lilienfeld and Evans, new definitions of epidemiology have been proposed. During this time, many fields related to epidemiology (pharmacoepidemiology, molecular epidemiology, genetic epidemiology…) have expanded. Moreover, epidemiology is related to many disciplines such as ethics , philosophy  and epistemology . These disciplines used existing definitions of epidemiology. Broadbend , in his work on philosophy and epidemiology, used the definition of epidemiology of Rothman and Last (p23) and insisted on the notion of comparisons of groups as did Morabia . Given the evolution in fields related to epidemiology, it is important to understand epidemiology and to analyse the evolution of definitions of epidemiology.
The main objective of this paper was to identify new definitions of epidemiology available since 1978, including veterinary medicine and epidemiology subspecialties [7, 8]. Secondary objectives were to analyse the content of these definitions, to compare them with those used by Lilienfeld and to determine whether changes have taken place over the last forty years.
Materials and methods
A review of grey literature and published literature was conducted to find definitions of epidemiology written between 1978 and 2017. It was conducted in English.
Grey literature on the subject was retrieved using the search engine "Google scholar” with the following keywords: (Definition AND Epidemiology) OR (definition of epidemiology). Definitions of epidemiology included in books were retrieved via Google books (https://books.google.fr/) and Amazon (http://www.amazon.fr/), Library of Congress (https://catalog.loc.gov/), The British Library (http://explore.bl.uk/primo_library/libweb/action/search.do?vid=BLVU1#) and NLM Catalog (https://www.ncbi.nlm.nih.gov/nlmcatalog/). Among the epidemiology textbooks identified, the book “Epidemiology and the people’s health: theory and context” by Nancy Krieger  contains a table with definitions of epidemiology from 1922 to 2007 retrieved from epidemiology textbooks. Works of Bhopal  and Zhang  were also considered as starting point. These definitions were selected as starting point for our work.
Definitions in the published literature were retrieved from "PubMed" and "ScienceDirect" using the following keywords: (Definition AND Epidemiology) OR (Definition of epidemiology) and the names of authors identified during the search of the grey literature.
Definitions for which the author could not be identified, those prior to 1978, those cited in Lilienfeld’s article  or those that could be related to the definition proposed by Lilienfeld or another author were eliminated.
Definitions provided by online dictionary sites, encyclopaedias like Wikipedia or internet sites dedicated to students were also retained, using the strategy mentioned above (keywords: (Definition AND Epidemiology) OR (definition of epidemiology)).
Analysis of the content of the new definitions
The content of new definitions of epidemiology was analyzed to identify the terms and concepts present in these definitions. Once a term or concept was identified in at least two definitions, it was added to the list of terms.
Analysis of the evolution of definition content between 1978 and 2017
In 1979, Evans  analyzed the content of the definitions used by Lilienfeld to build his definition of epidemiology. In summary, he identified 23 different terms and concepts. For each term and concept, the frequency of occurrence in the definitions was calculated. Then, they were grouped into eight categories: status of the person (disease, infectious disease, physiologic conditions, injuries, health); group affected (populations, community, mass phenomena, outbreak); distribution of disease; spread (spread, propagation, dynamics); incidence, occurrence; etiology (causes, determinant factors, circumstances of occurrence, ecology); understanding disease (natural history or nature, understanding the process); prevention and control.
To study the evolution of the content of definitions of epidemiology over time, the content of the definitions used by Lilienfeld was compared with new content from the period 1978–2017.
For each term and concept identified by Evans, the frequency of appearance in the definitions from the period 1978–2014 was estimated.
For each term and concept identified using the definitions from 1978 to 2017, the frequency of appearance in the definitions used by Lilienfeld was estimated.
Then for each term and concept, whatever the period, the frequency of appearance in the definitions used by Lilienfeld and definitions from 1978 to 2017 were compared.
All statistical comparisons were performed using bayes factors  with R software  (package BayesFactor). We have chosen bayes factor instead of p value, which is used in the context of null-hypothesis significance testing (NHST). Indeed, a nonsignificant p-value does not quantify evidence in favour of the null hypothesis. As stated by the American Statistical Association, “a relatively large p-value does not imply evidence in favor of the null hypothesis” .
The search of published and grey literature revealed 102 definitions of epidemiology: 93 for human medicine, including subspecialties of epidemiology (n = 24) and 9 for veterinary medicine. We have selected 29 definitions of epidemiology from websites. The definitions of epidemiology were found in different media: articles, epidemiology online courses and excerpts from books. A total of 69 definitions from 1978 to 2017 were selected (Table 1).
Analysis of the content of definitions from the period 1978 to 2017
Human medicine, general epidemiology
A total of 20 terms and concepts were identified in definitions from 1978 to 2017. Of the 20 terms and concepts, only three were not present in the definitions used by Lilienfeld: "knowledge" appeared in 1979 , "problems" appeared in 1991  and “public health” appeared in 1999  Table 2 shows the frequencies of appearance of terms in definitions from 1978 to 2017.
The term “population” or “group” was found in 84% of the definitions (58/69). For 25% of definitions (17/69), it was stated that epidemiology applies to human beings.
For 68% of the definitions (47/69), epidemiology was "the study of something" (study of …) and 17% of the definitions (12/69) defined epidemiology as a “science”. Four definitions (11%) used both terms [16–19].
Epidemiology was associated with “health” in 58% of definitions (40/69) and “disease” in 74% of definitions (51/69). Both terms were present in 23% of the definitions (16/69) [15–17, 20–32]. In 25% (17/69) of definitions, the term “disease” was not present but the terms “health problem” or “health states” was present [33–42] [43–49].
The concept of “health control” or “disease control” was present in 32% of the definitions (22/69). The concept of “disease prevention” or “prevention of health problems” was present in 16% of the definitions (11/69). These notions were associated in 9% (6/69) of the definitions [3, 17, 19, 39] [48, 50].
The term “problem” was present in 29% of the definitions (20/69). In 25% of definitions (17/69), the terms “problem” and “health” were associated (“health problem”) [15, 26] [28, 30, 32, 35, 37, 39, 42–49, 51].
Analysis of the evolution of definition content between 1978 and 2017
Table 3 shows a comparison of the content of definitions reviewed by Lilienfeld and those from 1978 to 2017 according to the concepts identified by Evans.
Several concepts (“infectious disease”, “mass phenomena”…) identified by Evans were not present in definitions from 1978 to 2017 (Table 3).
The term “health” was more frequent in definitions from 1978 to 2017 than in the definitions used by Lilienfeld (58% versus 4%, Bayes Factor(BF) = 18709.9).
The term “infectious disease” was less frequent in definitions from 1978 to 2017 than in the definitions used by Lilienfeld, with a BF estimated to 65.9 (0% versus 17%). "Mass phenomena/outbreak" and “infectious disease” can be related. The BF associated with "mass phenomena/outbreak, was estimated at 18, and this also reflects a decrease in the use of the term “infectious disease”.
The term “population” was more frequent in definitions from 1978 to 2017 than in the definitions used by Lilienfeld (84% versus 48%, Bayes (BF) = 63.1).
Table 4 shows a comparison of the content of definitions used by Lilienfeld and those from 1978 to 2017 according to the terms and concepts we have identified.
The term “problems” was probably more frequent in definitions from 1978 to 2017 than in the definitions used by Lilienfeld (29% versus 0%, BF = 43.8).
Human medicine, subspecialities of epidemiology
The following subspecialties (outcome-oriented or exposition-oriented) were taken into account, according to Boslaugh , Rothman and Carter-Pokras, , among many others: cancer epidemiology; cardiovascular epidemiology; obesity epidemiology; nutritional epidemiology; psychiatric epidemiology; genetic and molecular epidemiology; molecular epidemiology; genetic epidemiology; infectious diseases epidemiology; reproductive and perinatal epidemiology; environmental epidemiology; occupational epidemiology; social epidemiology. A total of 24 definitions were selected (Table 5).
The concepts mentioned in the definitions of subspecialty of epidemiology are similar to those found for general epidemiology. Some authors did not provided general definition of epidemiology [54–58], nor specific definition to subspecialty [59–61].
Others took up the main concepts of epidemiology and adapted them to their subspecialty. In this respect, the definition of the epidemiology of obesity speaks for itself: it contains virtually all concepts: “causes and consequences”; “human populations”; “the distributions, patterns, and dynamics”; “the determinants”; “to prevent and control”; “health conditions”. This definition also mentions a specificity of the subspecialty: “the development and validation of body composition measurement methods used in epidemiologic studies”.
Others referred to authoritative epidemiological literature or to WHO. For this case, the definitions of Last, 1995 and 2001 [43, 62] were most often cited (6 times out of 24 definitions), WHO (1994), Rothman and Greenland, 2008 , Susser, 1973  and Wikipedia being quoted once. Different subspecialties referred to the definition of Last: oncology , psychiatry , genetic epidemiology [66, 67], reproduction .
Finally, others Other authors proposed a pure and simple application of the general definitions of epidemiology to their subspecialty [64, 65]. Thus, Dos Santos after recalling Last's definition of epidemiology proposed a definition of cancer epidemiology and stated: “Therefore, this definition is as valid to cancer epidemiology as it is to epidemiology in general” . Tsuang defined psychiatric epidemiology as follow “Psychiatric epidemiology is simply the epidemiology of psychiatric disorders–no more, no less .
Some authors provided definitions by highlighting differences. This is the case, for example, for genetic epidemiology. Austin stated “Genetic epidemiology differs from epidemiology by its explicit consideration of genetic factors and family resemblance” .
Table 6 shows the frequencies of appearance of terms in definitions of epidemiology from 1977 to 2015, for veterinary medicine.
A total of 9 definitions were selected
Again, the concepts mentioned in the definitions of veterinary epidemiology are similar to those found for human epidemiology, with several authors using definitions proposed in human medicine and quoting for example MacMahon and Plug .
General epidemiology (websites)
The definitions of epidemiology are given in Table 7 and are classified according to the nature of the website: websites of international organizations, WHO, CDC, FAO (for veterinary medicine); national health institutes, academies of sciences …; public health schools; medical journals (BMJ); nonprofit organizations; Wikipedia; For-profit organizations; non-medical dictionaries online.
The distribution of concepts, by order of frequency, is the following: health (12/30); population (18/30); disease / illness (27/30); study (19/30); distribution (16/30). This distribution is almost that found with the definitions of epidemiology provided by books.
The majority of websites offer definitions of epidemiology already known, but does not cite the authors.
Few websites mentioned the date of the last update (e.g. National Institute on Deafness and Other Communication Disorders).
This study allowed us to identify, synthesize and analyse the definitions of epidemiology from the period 1978–2017. This evolution is almost absent from books on epidemiology and deserved to be presented. Even though many other fields of epidemiology have grown, only definitions of epidemiology were studied. A significant number of definitions of epidemiology were found despite the short period of study and the inclusion criteria. However, this number must be related to the increase in the number of publications in epidemiology.
Many of the books consulted and the definitions of epidemiology they contained summarized definitions that were already known  or definitions of other authors  or proposed mnemonics tools "the 3-D definition of epidemiology" [76, 77].
In summary, 69 definitions of epidemiology were retained from 1978 to 2017. All of these came from epidemiology textbooks or published articles. Only one definition (WHO) was not associated with a date of appearance . The fact that this definition was not cited by Lilienfeld was enough to keep it in this study.
The evolution of the content of definitions of epidemiology was studied on the basis of terms and concepts identified by Evans  and those identified by us.
The definitions from 1978 to 2017 helped to highlight 20 terms and concepts related to epidemiology. A great majority of concepts were either identified by Evans  or present in definitions used by Lilienfeld . Among the terms identified, only three (“problems”, “knowledge” and “public health”) were not present in Evan’s work. Five terms were present in more than 50% of definitions from 1978 to 2017: “population”, “study”, “disease”, “health” and “distribution”. These five terms can summarize epidemiology as being the study of the distribution of disease and health in the population. The term “study” may not seem specific enough for epidemiology. But this term is linked to the concept of “science”. Epidemiology was considered a “science” during both periods. In fact, the term “science” was used in more than 15% of definitions for the two periods.
Several developments have occurred. (i) the emergence of the term “problem” in the definitions of epidemiology; (ii) strengthening of the terms “control” and “health” already used; (iii) other terms and concepts (“infectious diseases”, “mass phenomenon”) identified by Evans  were no longer used in definitions from 1978 to 2017. But these terms were rare in the definitions used by Lilienfeld: 4/23 (17%), 1/23 (4%), 3/23 (13%) and 3/23 (13%), respectively.
The prevalence and distribution of diseases, infectious or not, also influence epidemiology. Thus, in low-income countries, new infectious diseases have emerged such as HIV infection. Nevertheless, the prevalence of these infectious diseases has declined over time. In low-income countries, infectious diseases are still present , but there is an increase in the incidence of chronic diseases . Epidemiological definitions have evolved in this direction, with a decrease in the presence of infectious disease terms.
Indeed, for definitions from 1978 to 2017, only one definition associated the terms “epidemiology” and “epidemic” . Currently, the term “epidemic” is associated with both infectious disease and the growing prevalence and incidence of disease states in the population, for example, obesity (obesity epidemic) . Thus, the concept of “disease” used in epidemiology is no longer limited to infectious diseases.
Epidemiology is currently associated with the study of disease, but also more broadly, with the study of health phenomena. In 23% (16/69) of the definitions retrieved, the concepts of “disease” and the concept of “health” were related. The term “disease” has a rather negative connotation, while the term “health” has a positive connotation. They reflect two opposing views of the same phenomenon. Some definitions suggested this phenomenon was dynamic and not static and evolved in the population, in space and in time [3, 33, 38, 82, 83].
In the definitions from 1978 to 2017, the link between epidemiology and public health was rare [16, 18, 20, 44, 84]. However, these definitions describe epidemiology as an essential part of public health. Other developments, nonetheless, were related to public health, like the expansion of the field of epidemiology to areas such as health promotion  and the use of statistical tools [3, 30, 32, 52].
Finally, 69 definitions (general epidemiology) were analyzed. This is greater than the number of definitions used by Lilienfeld. The fact that the definitions retained came from different sources, both grey and published literature, suggests that the definitions retained are representative of the available definitions of epidemiology, thus limiting the risk of selection bias. The evolution of the content of definitions of epidemiology was studied by using the concepts identified by Evans  and those identified by us. The comparison with the concepts cited by Evans allowed an objective analysis of the evolution of the content of definitions of epidemiology from 1978 to 2017. The statistical analysis revealed the differences observed between the two sets of definitions of epidemiology, even if the definitions of epidemiology from 1978 to 2017 were not exhaustively from this period.
The analysis of the evolution of the definitions of human epidemiology by subspecialty and veterinary epidemiology was not possible, due to the low number of definitions.
The number of subspecialties of epidemiology increases from year to year and almost all fields of medicine are concerned (). As early as 2007, Lillienfeld already raised the question of the general epidemiologist . As for general epidemiology, they are several definitions for each subspecialty. Austin , quoting Khoury's book published in 1993 , mentions that there were at least eight different definitions of genetic epidemiology at that time.
This increase in the number of definitions of epidemiological subspecialties can be linked to several factors including technological advances. The significant development of data analysis in epidemiology (epidemiological methods) could be related to the emergence of informatics. Molecular and genetic epidemiology is also undoubtedly linked to major advances in molecular and genomic biology . Advances in genome sequencing techniques have led to a better understanding of the genetic determinants of disease occurrence. They have therefore promoted the emergence of genetic epidemiology as a subspecialty of epidemiology , with a specific definition .
The prevalence of some cancers has also changed over time. Epidemiology has made it possible to identify some risk factors related to the environment and lifestyles. Nevertheless, the etiology of many types of cancer is still poorly understood. Indeed, it is generally accepted that most cancers result from the combined effects of environmental and genetic factors and that only a few cancers are only "of genetic origin" . The integration of molecular techniques into epidemiological studies has also led to the emergence of molecular epidemiology , with a definition proposed by Schulte et al. in 2012 .
Ecology and environment are a concern of human populations. The same applies to their impact on health. The emergence of environmental epidemiology is a particular reflection of this. The consideration of environment in the definition of epidemiology appeared in the 2000s, with the definition of Gerstman , which includes the term "modern" (with reference to old definitions). In previous years, only Cole, 1979  refers to the environment in his definition of epidemiology.
The digital revolution, linked to the rapid and unprecedented increase in the availability of data from various digital sources, has created new opportunities for collecting and analyzing data produced outside the health system. This led Salathé et al.  to introduce the term "digital epidemiology" and to propose a definition in 2018 . Some tools such as Google Trends are available and their use has led to mixed results .
The use of medical devices, whether implanted or not, has become crucial in many medical specialties for the diagnosis and treatment of certain diseases. Although their use is frequently beneficial to the patient, side effects may occur sometimes (e. g. infection on a vascular catheter). Monitoring these side effects and studying the risk factors for their occurrence is a necessity that has led to the individualization of the epidemiology of medical devices, with a specific definition [96, 97]. The methodological aspects specific to this field of epidemiology (definition of "real" exposure; choice of short- or long-term assessment criteria; various biases) were reported by Jalbert et al. .
Faced with the numerous specialties and the numerous books about on the topics, we had to make a selection. We recall that the main objective was general epidemiology, the description of the definitions of subspecialties of epidemiology being a complement.
The concepts found in the definitions of subspecialties of human epidemiology and those of veterinary epidemiology are generally the same. Consideration of economic aspects (productivity) is a specific feature of veterinary epidemiology.
The concepts found in the definitions of human epidemiology available on the websites are generally the same as those contained in the books or articles. Nevertheless, bibliographic references are often unavailable and access of information may change over time, with the disappearance of pages  or the existence of dead links ("error 404"…). The posting date is also rarely available.
Publications about the definitions of epidemiology exist. The fields and the aim are for example: introductory epidemiology textbooks ; description of definitions epidemiology over 50 years ; epidemiology and the methods needed for public health assessments ; evolution of epidemiological methods and concepts . But our review examines the evolution of definitions of epidemiology, taking into account more than 100 definitions of epidemiology retrieved from books or articles, introductory texts or not, general epidemiology or subspecialties, and a non-temporal description and analysis of the definitions of veterinary medicine. We also describe 29 definitions of epidemiology retrieved from websites.
This study has several limitations. Only English definitions were retained. National definitions of epidemiology not translated into English were eliminated . These definitions may have contained terms and concept related to epidemiology that were different from those in the definitions retained. But the probability that we have missed an important concept seems quite low. The terms and concepts identified depend on the content of the definitions selected. The definitions of epidemiology available in online dictionaries and other media were easy to find but the quality of the definitions was heterogeneous. To determine the quality of definitions, quality criteria need to be used. For the definitions from 1978 to 2017, the quality criteria may seems subjective. Moreover, no weights were assigned to definitions. The choice of weighting criteria may be subjective.
The purpose of the study was to identify concepts related to epidemiology and not to prioritize them. Given the variety of definitions, the fact that the method used to identify terms and concepts was easy to use and the fact that the majority of terms and concepts were present in more than 10% of definitions suggests that for the definitions retained all of the terms and concepts related to epidemiology were identified.
For future studies, we propose some weighting criteria such as inverse-variance weighting (meta-analysis…) which are widely accepted. We also propose that some of the following criteria be taken into account in the weighting grid: the number of concepts in the definition; the book type (collective dictionary as those of Last/ Porta; advanced texts; introductory texts).
In summary, this work led to a synthesis of different concepts related to epidemiology proposed during the period 1978–2017 and highlighted the evolution of the content of definitions of epidemiology over time. Most of the terms and concepts identified by us had already been used in the definitions of Lilienfeld while several terms and concepts identified by Evans were no longer used in definitions from 1978 to 2017. Increased usage of the terms “control” and “health” was found in definitions of epidemiology from 1978 to 2017. A thematic analysis of definitions of epidemiology could be conducted to complete this study, in order to improve our understanding of the changes observed.
The authors wish to thank Philippe Bastable for his assistance with editing the manuscript.
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