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Objectives
At the end of this lecture, the students will be able to:
• Define epidemiology.
• Describe the epidemiologic method.
• Identify the terms related to the epidemiology.
• Discuss the principles of epidemiology.
• Identify the core epidemiology functions.

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Definition of terms:
Health:
A state of complete physical, mental, and social well-being and not merely the
absence of disease or infirmity. (WHO)
Disease:
Any departure, subjective or objective, from a state of physiological or
psychological well-being.
Environment : Internal and external factors that constitute the context for
agent-host interactions.
Agent: A causative factor, such as a biological or chemical agent that must be
present (or absent) in the environment for disease occurrence in a
susceptible host.
Host: A person or living species capable of being infected.

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• Endemic:
- The constant presence of a disease or infectious agent within a given geographic
area or population group;
- It may also refer to the usual prevalence of a given disease within such area or
group.
• Epidemic:
- The occurrence of more cases of a disease than expected in a given area or
among a specific group of people over a particular period of time.
Pandemic :
An epidemic occurring over a very wide area (several countries or continents) at
the same time and usually affecting a large proportion of the population.
e.g. Influenza, cholera
Outbreak:
A localized epidemic affecting large number of a group, in the community
e.g. outbreak of food poisoning

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Epidemic period: - A time period when the number of cases of disease reported is
greater than expected.
Disease frequency: - Occurrence of disease as measured by various rates such as
morbidity rate.
Ecology: - the study of relations and interactions among all organisms within the
total environment.
Sporadic:
- Cases occur irregularly, haphazardly from time to time and generally
infrequently.
- Cases are few and separated widely in space and time showing no connection to
each other.

Elimination of disease:
• Disease incidence is reduced to a minimal level at which the disease is no
longer considered a public health problem, while infection may still occur
• Example:
• The aim of elimination of Neonatal tetanus is reduction of its incidence to
less than
one case/1000 live births.
Eradication:
• It means worldwide disappearance of a disease (permanent reduction to
zero level) with complete destruction of the agent.
• The organism can be present only in laboratories and no need for
interventions.
• e.g. smallpox eradication from the world since 1979.
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• Morbidity rate: A disease rate, specifically prevalence and
incidence rates of diseases in a population in a specified
time period.
• Mortality rate: The number of deaths from all causes
divided by the total population at a particular time and
place.
• Incidence rate: The rate of new cases of a condition or
disease in a population in a specified time period.
• Hypothesis: A supposition arrived at from observation or
reflection that leads to refutable predictions.

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•Rate: An expression of the frequency with
which an event occurs in a defined
population.
•Natural history of a disease: The course
that a disease would take from onset to
resolution without intervention by humans.
•Risk factors: it is a condition that is
associated with the increased probability of a
health related state or events.

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Definition of Epidemiology
Epi: upon
Demos: people or
population
Logo: study

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•Epidemiology is considered the basic science of
public health, and with good reason epidemiology is:
•a) a quantitative basic science built on a working
knowledge of probability, statistics, and sound
research methods;
•b) A method of causal reasoning based on developing
and testing hypotheses pertaining to occurrence and
prevention of morbidity and mortality; and
•c) A tool for public health action to promote and
protect the public’s health based on science, causal
reasoning, and a dose of practical common sense

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Definition of Epidemiology
Epidemiology is the study of the distribution &
determinants of DISEASE and health related events in the
community & the application of this study to prevent and
control of health problems.

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PRINCIPLES OF EPIDEMIOLOGY
The main objectives of epidemiology are to:
• Describe the occurrence of disease, and
• Explain the possible causes of disease
Describing a disease requires the gathering of information on
the distribution of disease in human populations based on
age, gender, race, and geographical area.

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1. Study
Epidemiology is a scientific discipline with
scientific foundation. Epidemiology is data-
driven and relies on a systematic and unbiased
approach to the collection, analysis, and
interpretation of data.

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2. Distribution
Epidemiology is concerned with the frequency and pattern
of health events in a population

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Frequency
Refers not only to the number of health events such as the
number of cases of meningitis or diabetes in a population,
but also to the relationship of that number to the size of
the population.

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Pattern
Time,
Place,
Person.

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• Time patterns may be (annual, seasonal, weekly, daily,
hourly, weekday versus weekend, or any other breakdown
of time that may influence disease or injury occurrence.
• Place patterns include geographic variation, urban/rural
differences, and location of work sites or schools. Personal
characteristics include demographic factors which may be
related to risk of illness, injury, or disability such as age,
sex, marital status, and socioeconomic status, as well as
behaviors and environmental exposures. Characterizing
health events by time, place, and person are activities of
descriptive epidemiology, discussed in more detail later

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3. Determinants
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.
To search for these determinants, epidemiologists
use analytic epidemiology or epidemiologic studies
to provide the “Why” and “How” of such events.

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Disease determinants
• Agent factors refer to an infectious microorganism—virus, bacterium,
parasite, or other microbes.
• They are necessary but not always sufficient alone to cause disease.
• Host factors are intrinsic factors that influence an individual’s
exposure, susceptibility, or response to a causative agent.( age,
nutrition, race,…
• Environmental factors are extrinsic factors which affect the agent and
the opportunity for exposure.

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• Epidemiological Triangle
• Agent factor: it refers to the cause of the disease, it
may include the infectious disease (bacteria, virus,
parasite, or other microbes), in non-infectious disease
like (chemical from food, radiation, insufficient food)
* NOTE: they are necessary but not always sufficient to cause
disease.

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Health-related states or events
• Epidemiology was originally focused exclusively on epidemics of
communicable diseases,
• endemic communicable diseases,
• non-communicable infectious diseases,
• chronic diseases,
• injuries,
• birth defects,
• maternal-child health,
• occupational health,
• and environmental health.
• Then epidemiologists began to look at behaviors related to health and
well-being, such as amount of exercise and seat belt use.
• Now, epidemiologists can make important strides in examining genetic
markers of disease risk.

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Specified populations
•The clinician is concerned about the health of an
individual;
•the epidemiologist is concerned about the people in
a community or population.
• In other words, the clinician’s “patient” is the
individual; the epidemiologist’s “patient” is the
community.

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4. 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.

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•Recently there are newer branches of epidemiology such
as:
• infectious disease epidemiology,
•chronic disease epidemiology,
•clinical epidemiology,
• cancer epidemiology,
• genetic epidemiology,
• neuro epidemiology,
• occupational epidemiology

Epidemiology is based on fundamental assumptions
•Human diseases are not distributed by chance in the
population.
•Distribution indicates something about how and why
disease process occurred.
•They have causal and preventive factors that can be
identified through systematic investigation of different
populations or same population in different places or at
different periods of time.

Difference between Clinical Medicine &
Public Health
Clinical Medicine Public Health
Aim Treatment of diseases Prevention of diseases
Target Individual patients Populations
Concern Diagnosis Assessment
Place Hospitals Community

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Types of epidemiology
1. Descriptive epidemiology
• Describing patterns and trends in health and disease in
populations
2. Analytical epidemiology
• Examining associations and causation
3. Experimental epidemiology
• Testing population interventions

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The objectives of epidemiology include the
following:
•To identify the etiology or cause of disease
•To determine the extent of disease
•To study the progression of disease
•To evaluate preventive and therapeutic measures
for a disease or condition
•To develop public health policy

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Core Epidemiologic Functions
•The major tasks of epidemiology in public health
practice are:
• Public health surveillance,
• Field investigation,
• Analytic studies,
• Evaluation,
• Policy development,

3 Types of epidemiological research:
1. Laboratory research: applies knowledge of basic sciences to
the development of procedures and strategies that enhance
our patho-physiological mechanisms.
2. Epidemic investigation: deal with outbreaks of disease in
specific populations, typically at the local level. Objectives are
to find the agent that caused the outbreak and its mode of
transmission and to suggest appropriate control measures.
3. Population research: called field or survey research: deals with
study of biological, environmental and behavioral determinants
of diseases

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