Definition and concept of Epedemiology.pptx

Basic Epidemiology
Sudhan Poudel
BPH, MPH
NHPC Reg no: 899 Jan PH

What is epidemiology?
Epidemiological approach:
Asking questions?
Making comparisons

Asking questions?
Related to Health Events:
What is the event? ( the problem)
What is its magnitude?
Where did it happen?
When did it happen?
Who are affected?
Why did it happen?

Asking questions?
Related to Health Action
1. What can be done to reduce the problem?
2. How can be prevented in future?
3. What action should be taken by community?
4. What resources required?
5. How activities to be organized?
6. What difficulties may arise?
Epidemiology is “a means of learning by asking questions
and getting answers that lead to further questions.”

Epidemiology ask?
Person- Who?
Place- Where?
Time- When?
Helps us to understand: Why?

Making Comparisons
 To find out the differences in the AGENT, HOST and
ENVIRONMENT conditions between two groups.
 Weighs, balances and contrasts give clues to ETIOLOGICAL
HYPOTHESIS.

Concept
Epidemiology is the study of distribution and determinants of
health related states or events in specified population, and the
application of this study to the control of health problems.

Understand the concept
Study:
Distribution : Who, where, when
Determinants: Why?
Health related states
Specified populations
Application

Study
 Study refers to surveillance, observation, hypothesis testing, analytic
research and experiments.
 Epidemiology is a scientific discipline with sound methods of scientific
inquiry at its foundation
 Epidemiological methods tend to rely on careful observation and use of
valid comparison groups to assess risk factors of health problems
 Epidemiology is data-driven and relies on a systematic and unbiased
approach to the collection, analysis and interpretation of data.

Distribution
• Epidemiology is concerned with the frequency and pattern
of health events in a population
• Frequency refers to the occurrence of number of health
events to the size of the population (Rate ,Ratio,
Proportion) The resulting value allows epidemiologists to
compare disease occurrence across different populations
• Pattern refers to the occurrence of health related events by
time, place and person also known by descriptive
epidemiology

Determinants
Determinants refers to the causes and other risk factors that
influence the occurrence of disease and other health-related events.
Illness or disease does not occur randomly in a population, but
happens only when the right accumulation of risk factors or
determinants exists in an individual
To search for these determinants, epidemiologists use analytical
epidemiological studies to identify the ‘Why” and “How” of such
events

Health related states or events
• Epidemiology was originally focused exclusively on epidemics of
communicable diseases
• It was subsequently expanded to address endemic communicable
diseases and non-communicable infectious diseases.
• In fact, the term health-related states or events may be seen as
anything that affects the well-being of a population

Population
Examines disease occurrence among population groups rather than among
individuals.
Epidemiology is often referred as “population medicine”.
The epidemiologic and clinical descriptions of a disease are quite difference.

Application
•Epidemiology is not just "the study of" health in a population; it
also involves applying the knowledge gained by the studies to
community-based practice.
•The epidemiologist uses the scientific methods of descriptive and
analytic epidemiology to understand local conditions in
"diagnosing" the health of a community and proposing
appropriate, practical, and acceptable public health interventions
to control and prevent disease in the community.

Introduction
• Epidemiology is the basic science of public health
that bridge between biomedical, social, and
behavioral sciences.
• The word “Epidemiology” is derived from Greek
words:
Epi = Among
Demos = People
Logos = Study, knowledge
The sciences which deals with what falls upon people…

Cont…
 Epidemiology is a scientific discipline with sound
methods of scientific inquiry
 Epidemiology is a quantitative discipline that relies on a
working knowledge of probability, statistics, and sound
research methods.
 Epidemiology is concerned with the frequency and
pattern of health events in a population

Cont…
 Epidemiology is also used to search for determinants,
which are the causes and other factors that influence
the occurrence of disease and other health-related
events.
 The primary unit of concern is a group of person not
any single person.
 Therefore epidemiology deals with population
pathology not the clinical pathology.

Definitions of Epidemiology
• It is branch of medical science that treats epidemics (Parkin,1873)
• Science of mass phenomenon of infectious diseases (Frost, 1927)
• “The study of the frequency, distribution and determinants of
disease in human population" (Mac Mohan and Pugh, 1960)

Cont..
• According to John M. Last, 1988 Epidemiology is
define as “the study of distribution and determinants
of health related states and events in specified
populations, and the application of this study to
control of health problems"

Aims of epidemiology
According to the International Epidemiological Association (lEA),
epidemiology has three main aims
1. To describe the distribution and magnitude of health and
disease problems in human populations
2. To identify etiological factors (risk factors) in the
pathogenesis of disease; and
3. To provide the data essential to the planning, implementation
and evaluation of services for the prevention, control and
treatment of disease and to the setting up of priorities among
those services.

Uses of Epidemiology
1. To identify causation of the disease
2. To study the natural history of the disease
3. To identify Health status of the population and
4. Evaluation of health care Interventions

1. Causation of the disease
• Most of diseases are caused by interaction between
genetic, environmental factors and personal
behaviors
• Epidemiology is used to study their influence and
the effects of preventive interventions through health
promotion.

Natural history of the disease
The natural history of disease is the course a disease takes in
individual people from its pathological onset until its eventual
resolution through complete recovery or death.
Epidemiology is also concerned with the course and outcome
(natural history) of diseases in individuals and groups.

3. Health status of the population
• Epidemiology is often used to describe the health
status of population.
• Knowledge of the disease burden in populations is
essential for health authorities.
• To use limited resources to the best possible effect by
identifying priority health programmes for
prevention and care.

4. Evaluation of Interventions
• To evaluate the effectiveness and efficiency of
health services.
• This means determining things such as –
- Impact of Contraceptive use on Population Control.
- the efficiency of sanitation measures to control diarrheal
diseases and
- the impact of reducing lead additives in petrol.

Applications of Epidemiology
Understanding disease occurrence in populations
Developing, prioritizing and assessing public health programs
Identifying risk factors for disease
Evaluating the efficacy of treatment options
Investigating disease outbreaks
Health planning and policy formation
Understanding the natural history of disease
Completing the clinical picture of disease

33
Infection
The entry and development or multiplication of an
Infectious agent in the body of man or animals. An
infection does not always cause illness.
• There are several levels of infection :
1. colonization (e.g., S. aureus in skin and normal nasopharynx);
2. subclinical or inapparent infection (e.g., polio);
3. latent infection (e.g., virus of herpes simplex);
4. Clinical infection.

34
Infestation
• For persons or animals the lodgment, development
and reproduction of arthropods on the surface of the
body or in the clothing, e.g., lice, itch mite.
• Some authorities use the term also to describe
invasion of the gut by parasitic worms e.g., ascariasis.

35
Infectious disease
• A clinically manifest (visible) disease of man or animals
resulting from an infection
• Contagious Disease: A disease that is transmitted
through contact. Examples include scabies, trachoma,
STD etc.

36
EPIDEMIC
• The "unusual" occurrence of disease or specific health-related
behavior (e.g., smoking) or other health-related events (e.g.,
traffic accidents) in a community or region that clearly in
excess of "expected occurrence”.
• The amount of disease occurring in the past, in the absence of
an epidemic, defines the "expected" frequency.
• Some use the term "outbreak" for a small, usually localized
epidemic in the interest of minimizing public sensation.

37
Endemic
• It refers to the constant presence of a disease or
infectious agent within a given geographic area or
population group, without importation (import) from
outside; may also refer to the "usual" or expected
frequency of the disease within such area or
population group.

38
• The term "Hyper endemic" expresses that the disease is
constantly present at a high incidence and/or prevalence rate
and affects all age groups equally;
• And the term "Holo endemic" a high level of infection
beginning early in life and affecting most of the child
population.
• An endemic disease when conditions are favorable may burst
into an epidemic (e.g.. hepatitis A. typhoid fever).

39
Sporadic
• The word sporadic means scattered about.
• The cases occur irregularly, haphazardly from time to time, and
generally infrequently.
• The cases are so few and separated widely in space and time that
they show little or no connection with each other, nor a
recognizable common source of infection, e.g., polio. Tetanus,
herpes zoster and meningococcal meningitis.

40
Pandemic
• An epidemic usually affecting a large proportion of the
population , occurring over a wide geographic area such as a
the entire nation, a continent or the world.
• E.g., influenza pandemics of 1918 and 1957, cholera in 1962
and acute hemorrhagic conjunctivitis in 1971 and 1981,
COVID pandemic 2019.

41
• Epizootic: An outbreak (epidemic) of disease in an animal
population (often with the implication that it may also affect human
populations), Only a few zoonotic agents cause major epidemics e.g.
brucellosis, rabies, influenza etc
• Enzootic: An endemic occurring in animals e.g., anthrax, rabies,
brucellosis, bovine tuberculosis etc
• Epornithic: An outbreak (epidemic) of disease in a bird
population.
• Exotic: Diseases which are imported into a country in which they do
not otherwise occur, as for example, rabies in UK.

42
Zoonoses
An infection or infectious disease transmissible under natural
conditions from vertebrate animals to man. May be enzootic or
epizootic – e.g., rabies, plague, bovine tuberculosis etc.
The term zoonoses has been further amplified as follows:
a) Anthropozoonoses : that is, infections transmitted to man from
vertebrate animals e.g., rabies, plague.
b) Zooanthroponoses : That is infections transmitted from man to
vertebrate animals e.g. human tuberculosis in cattle
c) Amphixenoses : That is infections maintained in both man and
lower vertebrate animals that may be transmitted in either
direction, e.g. T.cruzi, and S.joponicum.

43
Nosocomial Infection
• Nosocomial (hospital acquired) infection is an infection:
originating in a patient while in a hospital or other health care
facility.
• It denotes a new disorder (unrelated to the patient’s primary
condition) associated with being in a hospital.
• That is, it was not present or incubating at the time of admission or
the residual of an infection acquired during a previous admission.
• Examples include infection of surgical wounds, hepatitis B and
urinary tract infections.

44
Opportunistic infection
• This is infection by organisms that take the opportunity
provided by a defect in host defense (e.g. immunity) to infect
the host and thus cause disease.
• For example, opportunistic infections are very common in
AIDS.
• Organisms include Herpes simplex, cytomegalovirus, M.
tuberculosis.

45
Iatrogenic ( Physician induced) disease
• Any untoward or adverse consequence of a preventive,
diagnostic or therapeutic regimen or procedure, that causes
impairment, handicap, disability or death resulting from a
physician's professional activity or from the professional
activity of other health professionals.
• The disease may be serious enough to prolong the hospital
stay, require special treatment or actually threaten life.
• In short, iatrogenic disease is a hazard of health care.

Reservoir of infection
• Any person, animal, arthropod, plant, soil or substance
in which a infectious agent lives naturally and
multiplies on which it depends primarily for survival
and where it reproduces itself in such manner that it
can be transmitted to a susceptible host.

Cont..
• Incidence: Incidence refers to the number of
individuals who develop a specific disease or
experience a specific health-related event during a
particular time period (such as a month or year). (New
Cases)
• Prevalence: Prevalence refers to the total number of
individuals in a population who have a disease or
health condition at a specific period of time, usually
expressed as a percentage of the population. (New +
Old Cases)

48
Surveillance
• Surveillance has been defined as the continuous scrutiny of
the factors that determine the occurrence and distribution of
disease and other conditions of ill health.
• Surveillance is essential for effective control and prevention
and includes the collection, analysis, interpretation and
distribution of relevant data for action.
• The main purpose of surveillance is to detect changes in trend
or distribution in order to initiate investigative or control
measure.

49
Disease Control
• The term disease control describes ongoing
operations aimed at reducing
1. the incidence of disease
2. the duration of disease
3. the risk of transmission
4. the effects of infection: physical, psychological
5. financial burden to the community
• e.g. primary and secondary prevention
activities

50
Elimination
• WHO defines elimination as having no domestic transmission for the past three
years.
• Elimination is officially defined as reducing to zero the incidence of locally
acquired infection in a specific geographic area through deliberate efforts.
• The term elimination is used to describe interruption of transmission of disease.
• Between control and eradication , an intermediate goal has been described called
regional elimination.
• For example leprosy, measles, diphtheria etc.
• Disease elimination helps in disease eradication.

51
Eradication
• Eradication means to tear out by roots.
• Eradication of disease implies “Termination of all transmission
of infection by extermination(killing) of the infectious agent.”
• The word eradication is reserved to the end (stop) of infection
and disease from the whole world.
• For example small pox has been eradicated .
• Poliomyelitis, measles are in the process of eradication.

Rate
• Measures the frequency of an event in a population.
• Frequency of events, that occur in a defined time period,
divided by the average population.
• Eg. Crude birth rate, Crude death rate, Age specific rates etc

Numerator
Death Rate =
Number of death in one year
X 1000
Mid-Year Population
Rate comprises following elements:
•Numerator
•Denominator
Denominator
•Time specification
Time
•Multiplier
Multiplier

Ratio
• A value obtained by dividing one number by another
(either related or unrelated)
• Fraction that numerator is not a part of denominator.
• A ratio is the relative magnitude of two quantities
or a comparison of any two values.
• It is calculated by dividing one variable by the other.
• Eg. No of hospital bed per 100000 population, sex-
ratio, doctor-population ratio, child-woman ratio

Proportion
• A proportion is a ratio which indicates the relation in
magnitude of a part of the whole.
• It is a type of ratio in which the numerator is included
in the denominator.
• Numerator and denominator have the same units
(dimensionless), often expressed as percentage.
• Eg. Proportion of those who use modern contraceptives.

Case
• A person in the particular place or in a study, suffering from a
particular disease or disorder or condition under study/
investigation.
• Primary case: The individual who introduces the disease into
the family or group under study or the first case of
communicable disease, that introduces into a community under
study.
• Secondary case: An individual, who develops a disease after
being exposed with the primary case.
• Index case: The first case of communicable disease in a family
or other defined group under study, that comes to the attention
of the investigator.

Carrier
• An infected person or an animal that harbors a specific disease
agent with or without suffering from the disease, there may or may
not be recognizable clinical manifestation of the disease but they
are capable to transmit infection to others.
• Healthy carriers: These are the persons who harbor the
microorganism but had never suffered from any disease by it.
• Convalescent carriers: These are the persons who had been
infected by that microorganism and are recovering from that
infection. Even though they have no more symptoms of the disease,
they continue to shed the microorganism into the environment

58
Agent
The disease agent may be defined as a substance, living or nonliving or a
force tangible or intangible, excessive presence or relative lack of which
may initiate or perpetuate(responsible) a disease process.
1. Biological agents
2. Nutritive agents
3. Physical agents
4. Chemical agents:
a) Endogenous
b) Exogenous
5. Mechanical agents
6. Social agents

59
HOST
 A person or other animal, including birds and arthropods, that affords
subsistence (survival) or lodgment to an infectious agent under natural
(as opposed to experimental) conditions.
 An obligate host means the only host, e.g., man in measles
 Hosts in which the parasite attains maturity or passes its sexual stage are
primary or definitive hosts; those in which the parasite is in a larval or
asexual state are secondary or intermediate hosts.
 A transport host is a carrier in which the organism remains alive but does
not undergo development

60
Environment
• The term Environment implies all the external factors, living
and non living, material and non material, which surrounds
human.
• In its modern concept, environment includes not only the
water, air and soil that form our environment but also the
social and economic condition under which we live.
• Last (2001) defined the environment for the International
Epidemiological Association as: “All that which is external to
the human host, can be divided into physical, biological,
social, cultural etc and all of which can influence health status
of population.”

61
Vector
• Vector is defined as an arthropod or any living
carrier (e.g., Mosquito) that transports an
infectious agent to a susceptible individual.
• Transmission by a vector may be mechanical
or biological.

63
Screening
 Screening is a process/procedure applied to identify the presence
of a disease or a risk factor typically among asymptomatic
individuals
 Screening can be define as, a systematic and discipline process of
presumptive identification of unrecognised disease, disability or
defect among an apparently healthy or asymptomatic population
by the application of tests, examinations or procedures which are
simple, rapid, cheap and widely acceptable

64
Prevention
 Prevention includes a wide range of activities — known as “interventions”
— aimed at reducing risks or threats to health.
 Prevention can be define as following four level
 Primordial prevention: It includes intervention like identify and reduction of
risk factors of disease
 Primary prevention: It includes health promotion and specific protection
 Secondary prevention: It includes early diagnosis and treatment
 Tertiary prevention: It includes intervention like disability limitation and
rehabilitation

Terminologies in Epidemiology
Infection
The entry and development/multiplication of an infectious agent in the body of
man and animals. There is successful entry and growth of disease agents with
the production of changes in the host, which may be hypersensitivity/allergy,
disease, immunity
Infestation
The lodgement, development or reproduction of arthropods on the surface of the
body. Or the invasion of the gut by helminthes
Case
A person in the particular place or in a study, suffering from a particular disease
or disorder or condition under studied/ investigation.

Primary case
The individual who introduces the disease into the family or group under
study or the first case of communicable disease, that introduces into a
community under study.
Secondary case
An individual, who develops a disease after being exposed with the primary
case.
Index case..
The first case of communicable disease in a family or other defined group
under study, that comes to the attention of the investigator.
Carrier
An infected person or an animal that harbors a specific disease agent with or
without suffering from the disease, there may or may not be recognizable
clinical manifestation of the disease but they are capable to transmit infection
to others.

Agent
An agent is a substances, which may be living, non-living or a force which is
tangible or intangible whose presence, relative excess or relative lack is enough
for disease.
Host
A person or other animal including birds and arthropods, that subsistence or
provides lodgments to an infectious agent under natural conditions.
Environment
The aggregates of all those external and internal factors/forces, tangible or
intangible, living or non-living which is directly or indirectly related to the health
and survival of human beings or other living creatures
Sporadic
The cases or disease which occurs irregularly, haphazardly from time to time and
generally infrequently.
Endemic
It is usual frequency of a disease occurrence. The conditions in which there is
habitual presence of a disease or its agents within a given geographical area or
population group.

Epidemic
The unusual occurrence of disease in a community or a specific region in the
light of past experiences, it is the proportions of disease frequency beyond
expectation.
Pandemic
An epidemic usually affecting a large proportion of the population, it is the
occurrence or spread of an epidemic over a wide geographical area. Such as a
section of a nation, entire nation or worldwide.
Source/Reservoir
The person, animal or object or substances from which an infectious agent
pass or is disseminated to the host.
Surveillance: It is an ongoing systematic collection, collation, analysis and
interpretation of data and dissemination of information to who need to know
in order that action may be taken.

Opportunistic infection
It is the infection by agent/organism that takes the opportunity provided by an alteration in
host defense mechanism and hence cause disease. For example opportunistic infection are
very common in AIDS.
Eradication
It is the process or state in which there is termination of all transmission of infection by
extermination of infectious agent through the surveillance system and containment.
Contagious Disease
The disease/infection that is transmitted through the contact like scabies, leprosy, trachoma.
Communicable Disease
An illness due to specific infectious agent or its toxic products capable of being directly or
indirectly transmitted to man to man or animal to animal or from the environment to man
or animal.
Epizootic: An outbreak of disease in animal population - anthrax, rabies, brucellosis
Epornithic: An outbreak of disease in birds population

Incubation period
The time interval between invasion of microorganism and the development of first
sign/symptoms of a disease cause by the same microorganism. Incubation is some
time also called generation time.
Generation time
The time interval between receipt of infection by a host and maximal infectivity of
that host.
Latent infection
The condition in which the host does not shed the infectious agent that lies
dormant within the host without symptoms
Nosocomial infection
Nosocomial infection (hospital acquired infection) is an infection originating in a
patient while in a hospital or other health care facility. It denotes a new disorders
(unrelated to the patient’s primary condition) associated with being in a hospital.
Iatrogenic (physician induced) disease
Any untoward or adverse consequences of a preventive, diagnostic or therapeutic
regimen or procedure, that causes impairment, handicap, disability or death
resulting from a physician’s professional activity or from the professional activity
of other health professionals.

Cont…
Risk/risk factors: Risk is the probability of getting disease or condition. Increase
probability of getting certain disease when exposed to certain factors is known as
risk factor or the factors whose presence or absence is associated with an
increased likelihood of getting disease
Surveillance: It is an ongoing systematic collection, collation, analysis and
interpretation of data and dissemination of information to who need to know in
order that action may be taken.
Screening: It is a systematic and discipline process of presumptive identification
of unrecognized disease, disability or defect among an apparently healthy or
asymptomatic population by the application of test, examinations or procedures
which are simple, rapid, cheap and widely acceptable

Historical development of epidemiology
The history of epidemiology has its origin in
the idea goes back to (400 B.C)
Hippocrates Who suggest that environment &
human behaviors affects health.
This represented a movement away from
supernatural explanations of disease causation

Cont…
The foundation of studying the epidemiological
measurements was started during 16th
century by a
amateur scientist called John Graunt (1620- 1674), who
was a professional haberdasher.
In 1662 John Graunt published a landmark analysis
of Mortality data.
It was the first quantify patterns of birth, death, and
disease occurrence.

Cont..
In 1800 – William Farr work by
systematically collecting and analyzing
Britain's mortality statistics.
Farr, considered the father of modern vital
statistics and surveillance, developed many of
the basic practices used today in vital
statistics and disease classification.

Cont….
John Snow was famous for the suppression of an
1854 outbreak of cholera in London’s Soho district
John Snow was conducting a series of
investigations for prove that disease is not due to
miasma (miasmatic theory)
Twenty years before the development of the
microscope, Snow conducted studies of cholera
outbreaks both to discover the cause of disease and
to prevent its recurrence.
In 1854 -John Snow , was considered as the
“father of field epidemiology.

Snow’s Contributions
 He prepared a spot map and marked the location of pumps,
and than he observed the relationship between the
distribution of cholera and infected households and the
location of the pumps.
 Snow noted an apparent association between the source of
drinking water and the deaths, by a statistical comparison of
cholera deaths in district with different water suppliers
 Linked the cholera epidemic to contaminated water supplies.

• Spot map: Map showing the geographic location of people with a specific
attribute, cases of an infectious disease. The spot map below focuses on "place" by
showing where each of the known cases lives. Spot map helps to formulate
hypothesis by providing clear picture of distribution of known cases.

Cont..
In 19th and 20th centuries epidemiological methods applied in
investigation of disease occurrence , at that time, most investigators
focused on acute infectious diseases.
 In 1930s &1940s non-infectious diseases were also investigated
for their risk-factors.
In 1954 Publication of the result of a British Doctors study, led
by Richard Doll and Austin Bradford Hill, which lent very strong
statistical support to the suspicion that tobacco smoking was linked
to lung cancer

Cont…
The study of heart disease among residents of Framingham,
popularly known as one of a classical cohort study, it beings since
1949 to investigates coronary heart disease risk factors
Later in 1965, the British statistician Sir Austin Bradford Hill
proposed a set of features that should be sought when deciding
whether a relationship between cause and effect are causal or non-
causal association (popularly known as Hill’s criteria for
establishing causal inference)

Cont…
 Finally, during the 1960s and early 1970s application of
epidemiological methods/interventions got succession to
eradicates smallpox world wide in 1980.
 In late 1900s Rothmans discovery of necessary cause and
sufficient cause which strengthened the multifactor disease
causation concept in contrast to other concepts of disease
causation i.e germ theory, ecological theory etc.

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