Subject, Theory, Methods Subject, Theory, Methods and Tasks of Epidemiologyand Tasks of Epidemiology

Prof. Dr. N. Ribarova, MDProf. Dr. N. Ribarova, MD

Medical science as a whole encompasses a Medical science as a whole encompasses a great number of formal sciences which are great number of formal sciences which are estimated in two major trends, prophylaxis estimated in two major trends, prophylaxis and therapy, depending on their subjects and and therapy, depending on their subjects and aims.aims.

Prophylactic disciplines have as their Prophylactic disciplines have as their subject to study subject to study healthy manhealthy man, and their aim , and their aim is is to preserve human healthto preserve human health by studying the by studying the causes of diseases and their terms of causes of diseases and their terms of emergence as well as the ways of preventing emergence as well as the ways of preventing them.them.

Epidemiology is one of the main Epidemiology is one of the main disciplines in this group.disciplines in this group.

The development of epidemiology of The development of epidemiology of infectious diseases during the centuries have infectious diseases during the centuries have been governed by public health needs.been governed by public health needs.

The definition of epidemiology as a science The definition of epidemiology as a science in various historical periods has relied on the in various historical periods has relied on the relative authenticity of social phenomena and relative authenticity of social phenomena and processes.processes.

Till the middle of XX century many Till the middle of XX century many authors define authors define epidemiologyepidemiology as a science for as a science for study and fight against study and fight against epidemic diseases.epidemic diseases.

Progress in infectology later defines it as a Progress in infectology later defines it as a science of the science of the epidemic processepidemic process; most ; most recently, it has been noted that this definition recently, it has been noted that this definition does not reveal its essence as well.does not reveal its essence as well.

WHO – 1960 – Prague:WHO – 1960 – Prague:

Epidemiology is an independent trend of Epidemiology is an independent trend of medical science studying the causes for the medical science studying the causes for the emergence and dissemination of infectious emergence and dissemination of infectious diseases in human society and applying thus diseases in human society and applying thus gained knowledge for fight against, gained knowledge for fight against, restriction and, eventually, complete restriction and, eventually, complete eradication of these diseases.eradication of these diseases.

Epidemiology is a scientific area studying Epidemiology is a scientific area studying infectious diseases in respect to evolution and infectious diseases in respect to evolution and ecology as well as in relation to antiepidemic and ecology as well as in relation to antiepidemic and prophylactic issues; it studies their biosocial prophylactic issues; it studies their biosocial nature and dependence on social and natural nature and dependence on social and natural factors.factors.

Epidemiology realizes various aims including Epidemiology realizes various aims including eliminationelimination and and liquidation liquidation of infectious of infectious diseases as well.diseases as well.

Epidemiology of infectious diseasesEpidemiology of infectious diseases is a is a scientific area with broad frames of scientific area with broad frames of knowledge in the field of existence, knowledge in the field of existence, occurrence and disappearance of parasitic occurrence and disappearance of parasitic pathology in human society.pathology in human society.

Epidemiology of infectious diseasesEpidemiology of infectious diseases, as a , as a separate medical science, is a fundamental separate medical science, is a fundamental science within prophylactic medicine science within prophylactic medicine because it studies and offers prophylactic because it studies and offers prophylactic measures for mass diseases. It has one of the measures for mass diseases. It has one of the basic activities in medical science and basic activities in medical science and practice.practice.

Subject of EpidemiologySubject of EpidemiologyAt the current stage of development of At the current stage of development of

knowledge, the subject of epidemiology is knowledge, the subject of epidemiology is defined as the state of interactions between a defined as the state of interactions between a microorganismmicroorganism and a and a microorganismmicroorganism on on individual and population level, at specific individual and population level, at specific social and ecosystemic conditions – or the social and ecosystemic conditions – or the epidemic processepidemic process, notwithstanding the , notwithstanding the theories of many authors specifying a series of theories of many authors specifying a series of forms, states, and factors in the development forms, states, and factors in the development of infectious diseases in human society which of infectious diseases in human society which do not find their place within the definition of do not find their place within the definition of epidemic process.epidemic process.

Theory of EpidemiologyTheory of Epidemiology

As any independent medical science, epidemiology As any independent medical science, epidemiology has its theory. Having been developed in the course of has its theory. Having been developed in the course of historical and logical development of knowledge, it historical and logical development of knowledge, it encompasses the following trends:encompasses the following trends:►Principles of epidemic process;Principles of epidemic process;►Principles of infectious process;Principles of infectious process;►Theory of immunity and use of specific Theory of immunity and use of specific immunoprophylaxis;immunoprophylaxis;►Principles of natural focal diseases;Principles of natural focal diseases;►Theory of elimination and liquidation of infectious Theory of elimination and liquidation of infectious diseases;diseases;

►Theory of the systemic character of epidemic Theory of the systemic character of epidemic process and systemic approach for its study and process and systemic approach for its study and prevention;prevention;

►Scientific system of epidemic surveillance and Scientific system of epidemic surveillance and control of infectious diseases;control of infectious diseases;

►Scientific system of infectious diseases Scientific system of infectious diseases prevention;prevention;

►Organizational foundations of the activities for Organizational foundations of the activities for infectious diseases prevention and control;infectious diseases prevention and control;

►Epidemiological nosogeography.Epidemiological nosogeography.

Aim of EpidemiologyAim of Epidemiology

To study the laws and dissemination of To study the laws and dissemination of infectious diseases; to organize the infectious diseases; to organize the epidemiological practice of the entire society epidemiological practice of the entire society for their restriction, prevention of for their restriction, prevention of complications, reduction of economic and complications, reduction of economic and social damages and, in the final account, social damages and, in the final account, elimination and liquidation of these diseases.elimination and liquidation of these diseases.

Goals of the epidemiologyGoals of the epidemiology

1.1. To explain theoretically the causes for arising, To explain theoretically the causes for arising, development and expirerius of the epidemical development and expirerius of the epidemical processes.processes.

2.2. To create and apply effective means and methods To create and apply effective means and methods for control of the infectious diseases.for control of the infectious diseases.

3.3. To create and apply effective means and methods To create and apply effective means and methods for prevention of infectious diseases, encluding for prevention of infectious diseases, encluding their elimination and liquidation. their elimination and liquidation.

History of EpidemiologyHistory of EpidemiologyWithout studying the history of the subject Without studying the history of the subject

of a science, a theory of the discipline and of a science, a theory of the discipline and logical reflection of the nature of the processes logical reflection of the nature of the processes in the different stages of its development in the different stages of its development cannot exist.cannot exist.

The historical development of knowledge on The historical development of knowledge on epidemiology has progressively enriched its epidemiology has progressively enriched its theory.theory.

Stages of development of the knowledge on the Stages of development of the knowledge on the epidemiology of infectious diseases:epidemiology of infectious diseases:►Empirical speculations on the contagiousness of Empirical speculations on the contagiousness of

infectious diseases;infectious diseases;►Enrichment of the knowledge on the causes, Enrichment of the knowledge on the causes,

emergence and dissemination of epidemic diseases emergence and dissemination of epidemic diseases and enlargement of the empirical measures to fight and enlargement of the empirical measures to fight these diseases;these diseases;►Emergence and disappearance of an infectious Emergence and disappearance of an infectious

disease during the Middle Ages;disease during the Middle Ages;►Confirmation of the principles of the Confirmation of the principles of the

contagiousness of infectious diseases. Beginning of contagiousness of infectious diseases. Beginning of specific vaccine prophylaxis;specific vaccine prophylaxis;

►Development of bacteriology and the theory of Development of bacteriology and the theory of special epidemiology;special epidemiology;

►Empirical data showing alterations in the severity of Empirical data showing alterations in the severity of the course of infectious diseases;the course of infectious diseases;

►Emergence of the idea for a public stand on the fight Emergence of the idea for a public stand on the fight against infectious diseases;against infectious diseases;

►Establishment of the principles of epidemic process;Establishment of the principles of epidemic process;►Implementation of the program for eradication of Implementation of the program for eradication of

malaria in the 1950s;malaria in the 1950s;►Molecular epidemiology: molecular-biological and Molecular epidemiology: molecular-biological and

molecular-genetic methods for diagnosing infectious molecular-genetic methods for diagnosing infectious diseasesdiseases

Method of EpidemiologyMethod of Epidemiology

The The epidemiological methodepidemiological method is an aggregate of is an aggregate of methodological know‑how – observation, methodological know‑how – observation, historical and geographical description, historical and geographical description, juxtaposition, statistical analysis, diagnostic juxtaposition, statistical analysis, diagnostic methods designed to characterize the epidemic methods designed to characterize the epidemic process as well as to substantiate the measures for process as well as to substantiate the measures for prevention and control of infectious diseases and prevention and control of infectious diseases and to estimate their effectiveness.to estimate their effectiveness.

The method is specific for the epidemiology of The method is specific for the epidemiology of infectious diseases because it studies the triad:infectious diseases because it studies the triad:►parasiteparasite – in biological, ecological and – in biological, ecological and

epidemiological aspect;epidemiological aspect;►macroorganismmacroorganism – reaction during the – reaction during the

encounter with the parasite;encounter with the parasite;►environmentenvironment – in all its aspects with the aim to – in all its aspects with the aim to

develop and apply prophylactic programs for develop and apply prophylactic programs for liquidation and elimination of infectious diseases.liquidation and elimination of infectious diseases.

Parts of the epidemiological method:Parts of the epidemiological method:

1. Universal part:1. Universal part:► Definition of the problem;Definition of the problem;► Collection, classification and analysis of the Collection, classification and analysis of the

information;information;► Formulation of a hypothesis and determination of the Formulation of a hypothesis and determination of the

hypotheses subject to testing;hypotheses subject to testing;► Testing the formulated hypothesis – through prospective Testing the formulated hypothesis – through prospective

and longitudinal studies;and longitudinal studies;► Determination of specific factors leading to emergence Determination of specific factors leading to emergence

of a particular disease in the group of highly exposed of a particular disease in the group of highly exposed individuals;individuals;

► Conclusions and subsequent control and preventive Conclusions and subsequent control and preventive measures. Estimation and testing their effectiveness.measures. Estimation and testing their effectiveness.

2. Specific part:2. Specific part:►Studying the biology, ecology and immunology Studying the biology, ecology and immunology

of the specific etiological agent;of the specific etiological agent;►Studying the epidemiology of the infectious Studying the epidemiology of the infectious

disease;disease;►Creation of a scientific theory of the nature of Creation of a scientific theory of the nature of

the infectious disease and the fight against such the infectious disease and the fight against such a disease;a disease;

►Setting up a program for prevention;Setting up a program for prevention;►Practical application of the program for Practical application of the program for

prevention and acquisition of the assigned aim.prevention and acquisition of the assigned aim.

The logical sequence of know-how skills The logical sequence of know-how skills utilized by the epidemiological approach helps to utilized by the epidemiological approach helps to the formulation of the following methods:the formulation of the following methods:►Descriptive;Descriptive;►Analytical;Analytical;►Experimental;Experimental;►Mathematical modeling.Mathematical modeling.

Descriptive methodDescriptive methodAimAim – to determine the incidence of the disease, – to determine the incidence of the disease,

the risk factors and its medicosocial significance the risk factors and its medicosocial significance and to formulate the and to formulate the hypothesishypothesis for the possible for the possible prevention and control measures.prevention and control measures.

Baseline data:Baseline data: the registered and reported the registered and reported materials and the results from the representative materials and the results from the representative studies.studies.

The The registered and reported materialsregistered and reported materials are are characterized by accessibility and ease of characterized by accessibility and ease of collection.collection.

The The representative studiesrepresentative studies are more precise are more precise but more complex for performance.but more complex for performance.

Representativeness is a variable determined by means of an estimation Representativeness is a variable determined by means of an estimation of the variation series.of the variation series.

Variants of representative studies utilized in the epidemiologyVariants of representative studies utilized in the epidemiology

Signs Variants

Time of performance Single-point (cross-section) study.

Long-term (longitudinal, prospective) study

Method of forming representativeness

Individual selectionCluster selectionStratified selection

Parameters of the descriptive method:Parameters of the descriptive method:►IncidenceIncidence – the ratio between the number of – the ratio between the number of

newly registered individuals with an infectious newly registered individuals with an infectious disease for a specified period of time and the disease for a specified period of time and the individuals at risk of developing the disease individuals at risk of developing the disease during the same period in %during the same period in %000000..

►PrevalencePrevalence – the ratio between the number of – the ratio between the number of cases with an infectious disease (newly cases with an infectious disease (newly registered and old cases) at a definite point of registered and old cases) at a definite point of time and the magnitude of the population at risk time and the magnitude of the population at risk at the same point in %at the same point in %00..

►MortalityMortality – the ratio between the number of – the ratio between the number of deaths for a specified period and the number of deaths for a specified period and the number of individuals at risk of death for the same period x individuals at risk of death for the same period x 100 000.100 000.

►LethalityLethality – the ratio between the number of – the ratio between the number of individuals deceased from a particular disease for individuals deceased from a particular disease for a specified period and the number of diseased a specified period and the number of diseased individuals during the same period in %.individuals during the same period in %.

► Incidence in respect to particular variables:Incidence in respect to particular variables:* age;* age;* gender, education, socioeconomic status;* gender, education, socioeconomic status;* profession;* profession;* territorial characterization;* territorial characterization;* isolated microorganisms;* isolated microorganisms;* variability;* variability;* seasonal variations;* seasonal variations;* focal variations;* focal variations;* index of effectiveness of the measures.* index of effectiveness of the measures.

Statistical parametersStatistical parametersIn the epidemiological studies, all available to In the epidemiological studies, all available to medical statistics generally accepted statistical medical statistics generally accepted statistical parameters are utilized:parameters are utilized:- absolute values (numbers);- absolute values (numbers);- intensive parameters;- intensive parameters;- average values;- average values;- extensive parameters;- extensive parameters;- cumulative data;- cumulative data;- relative values;- relative values;- ratio parameters;- ratio parameters;- standardized parameters.- standardized parameters.

The reliability of statistical parameters and the The reliability of statistical parameters and the relation among the studies epidemiological events relation among the studies epidemiological events is subject to estimation. Leveling of dynamic is subject to estimation. Leveling of dynamic series and estimation of the direction and series and estimation of the direction and occurrence of trends is performed.occurrence of trends is performed.

Descriptive epidemiology is occupied with Descriptive epidemiology is occupied with collection of data for the health status of the collection of data for the health status of the population and its change over population and its change over timetime with the aim with the aim to establish the foundation for organizational to establish the foundation for organizational measures, planning and management of medical measures, planning and management of medical aid as well as formulation of hypotheses on aid as well as formulation of hypotheses on possible possible cause-effectcause-effect relations. relations.

The The descriptive methoddescriptive method of epidemiology is of epidemiology is used for analyzing the characteristic features of the used for analyzing the characteristic features of the distribution of the incidence of a specific distribution of the incidence of a specific nosological entity according to territory among nosological entity according to territory among various social and age populations and specific various social and age populations and specific groups, and according to time (multi-annual and groups, and according to time (multi-annual and annual dynamics, cyclic and seasonal variations) annual dynamics, cyclic and seasonal variations) expressed in intensive parameters.expressed in intensive parameters.

The formulation of the hypothesis for the risk The formulation of the hypothesis for the risk factors utilizes factors utilizes logical methodslogical methods to discover to discover causal causal relationsrelations.. Quantitative methods Quantitative methods are used for are used for statistical estimation of the relations between statistical estimation of the relations between epidemiological phenomena. This method does not epidemiological phenomena. This method does not clarify the character of these relations which is clarify the character of these relations which is done through the analytical method.done through the analytical method.

Analytical methodAnalytical method

It discovers the causal relation between the It discovers the causal relation between the disease and the external factors; it estimates and disease and the external factors; it estimates and confirms the hypothesis for the risk factors causing confirms the hypothesis for the risk factors causing the emergence of the problems discovered with the the emergence of the problems discovered with the help of the descriptive epidemiological method; it help of the descriptive epidemiological method; it forms the theory of epidemiology and the forms the theory of epidemiology and the prevention of diseases.prevention of diseases.

Major analytical trends of research are:Major analytical trends of research are:►Cohort studyCohort study – an analytical research study in which – an analytical research study in which

the groups of individuals are defined on the basis of the groups of individuals are defined on the basis of the presence or absence of exposure to a probable risk the presence or absence of exposure to a probable risk factor, and subsequently the groups are compared in factor, and subsequently the groups are compared in relation to the proportions within these groups who relation to the proportions within these groups who have developed the studied disease.have developed the studied disease.

►Case-control studyCase-control study – a type of analytical research – a type of analytical research study in which subjects are selected on the basis of study in which subjects are selected on the basis of whether they have (case) or have not (control) a whether they have (case) or have not (control) a particular disease. Subsequently the groups are particular disease. Subsequently the groups are compared in relation to the proportions within these compared in relation to the proportions within these groups who have been exposed to a specific risk groups who have been exposed to a specific risk factor.factor.

Experimental methodExperimental methodIt is applied only in cases of It is applied only in cases of controlled controlled

reduction of exposition.reduction of exposition.Experimental epidemiology is closely related Experimental epidemiology is closely related

with primary prevention of the disease. In many with primary prevention of the disease. In many cases, the setting up of a program for prevention is cases, the setting up of a program for prevention is much more a question of applying existing much more a question of applying existing knowledge than gathering new information.knowledge than gathering new information.

Experimental epidemiological studies can be Experimental epidemiological studies can be performed with a group of individuals where a performed with a group of individuals where a suspected factor is influenced by means of various suspected factor is influenced by means of various diagnostic, therapeutic and prophylactic diagnostic, therapeutic and prophylactic interventions, and the effect of their influence is interventions, and the effect of their influence is compared among 2 or more experimental and compared among 2 or more experimental and control groups.control groups.

Mathematical modelingMathematical modeling

During the last decades, epidemiological models During the last decades, epidemiological models have been developed to be used for the study of the have been developed to be used for the study of the natural course of the epidemic process in a particular natural course of the epidemic process in a particular disease as well as for the study of the perspectives of disease as well as for the study of the perspectives of prevention and control in view of the elimination and prevention and control in view of the elimination and liquidation of infectious diseases.liquidation of infectious diseases.