Measurement of health and disease

INTRODUCTION TO EPIDEMIOLOGY

B Y A L E M N E W D E S TAW ( M P H , P H D F E L L O W )
Session 1: General Principles of Epidemiology
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 History, definition, uses, scope, and assumptions

 Disease causation: concepts and factors

 Models: Epidemiological triangle, web of causation,

wheel model

 Time, Place and Person concept in disease causation

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 Learning Objectives
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By the end of this session, students should be able to:

 Define epidemiology

 Describe the history of epidemiology

 Explain the basic assumptions of epidemiology

 Identify the scope of epidemiology

 Describe basic models of disease causation.

 Identify the classification of epidemiology

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Definition
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Epidemiology is the study of the

 frequency,

 distribution, and

 determinants of health-related states

in specified populations, and the application of this

study to the control of health problems.

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 Definition…
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 Is concerned with the collective health of individuals

in communities and in getting appropriate solution to

alleviate the health problems.

 It provides useful tools and methods

 to describe variations in disease occurrence and

 identify factors that influence the occurrence of disease among

population groups.

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 Terms from the definition
Study: Epidemiology is6 a scientific discipline,
sometimes called “the basic science of public health”.

Frequency : Epidemiology is a quantitative science.

Distribution: refers to the occurrence of health-related

events by time, place, and personal characteristics.

Determinants: used to search for causes and other

factors that influence the occurrence of health-related

events.
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Health-related states or events.
 Endemic communicable diseases
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and non communicable
diseases.
 Chronic disease, injuries, birth defects, Maternal and
child health, occupational health, and environmental
health.
 behaviors related to health and well-being (eg. amount of
exercise, seat-belt use, etc.)

 Thus ,we use the term “disease” to refer the range of

health-related states or events.

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 Application: Epidemiology Provides the base for
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directing practical and appropriate public health action.
“Epidemiology is practical science”

 Specified populations: Focus on the population.

 Epidemiologists and physicians in clinical practice differ

greatly in how they view “the patient.”

 Clinician usually focuses on diagnosing & treating the

individual.

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The epidemiologist focuses on assessing ;
 The level of health related event in
9 the community

 The number of other persons who may have been similarly exposed to
the factor

 The type and source of exposure to the factor

 The potential for further spread of the disease in the community

 Interventions to prevent additional cases or recurrences of the problem.

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 Epidemiology Classification
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1. Descriptive epidemiology - describing the frequency
and distribution of diseases and other health related
conditions by Person, Place and Time (PPT)
 It answers the questions Who, Where and When.

 Person (Who were affected? )

Young Vs Old
Female Vs Male
Rich Vs Poor
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Descriptive cont‟d
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Place (Where was the problem occurred? )

 Lowland Vs Highland

 Urban Vs Rural

Time ( When was the problem occurred? )

 Seasonal Variations

Long term variations

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2. Analytic epidemiology
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 Concerned with identification of causes and other

factors that influence the occurrence of health-related

events in the population

 It answers the questions WHY and HOW.

 Involves explicit comparison of groups of individuals

for the level of health related event to identify

determinants of health and diseases.

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 Descriptive and analytic 13classification is more of a

continuum than a dichotomy. They go together.

 Many studies have both descriptive and analytic

aspects.

 Data collected in one mode may end up being used in

the other as well.

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 History of Epidemiology
Hippocrates (400 B.C.): 14
 attempted to explain disease rationally rather than
through supernatural beliefs.
 Wrote the essay „„On Airs, Waters, and Places‟‟:
 Emphasized the role of environmental factors
(like air, water, and living conditions) in disease
causation.
 Considered as the first epidemiologist

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 John Graunt (1662)
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 The most important advances in epidemiology are

attributed to the Englishman John Graunt (1620–1674).

 He was the first epidemiologist to quantify patterns of

birth, death, and disease occurrence

 Observed male-female disparities, high infant

mortality, urban-rural differences, and seasonal
variations.

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 James Lind (1747)
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 First person to conduct an experimental study in human

populations.

 In 1747, conducted a controlled trial aboard a British

naval ship.

 Identified that scurvy (gum bleeding) was caused by a

deficiency of citrus fruits (vitamin C).

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 Edward Jenner (1749-1823)
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 Pioneered the smallpox vaccine.

 His work laid the foundation for vaccination programs

and contributed to the eradication of smallpox in the 20th

century.

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 William Farr (1839-1883)
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Father of Modern Vital Statistics:
 Established the use of vital statistical data for evaluating health
problems.
 Developed concepts like population at risk and choosing
appropriate comparison groups, still in use today.
Extended John Graunt's Work:
 Systematically collected and analysed Britain‟s mortality
statistics.

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John Snow (1853):The First Modern Epidemiologist

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 Formulated and tested a hypothesis about the origin of a
cholera epidemic in London.
 Postulated that cholera was transmitted by contaminated water.

 During the cholera outbreak in Golden Square, London, Snow

investigated where cholera patients lived and worked.
 Then he mapped the locations of cholera cases on a spot map.

 Identified the source of the outbreak as a specific water pump

on Broad Street.

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 Scope of Epidemiology
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Currently Epidemiology deals with health related
problems whether they belong to;

Communicable,

Non-communicable Or

Any Type Of Injury Category.

Its scope in public health ranges from routine

surveillance to research strategies

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 Use/applications of Epidemiology
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Elucidate the natural history of disease.

Describe the health status of the population.

 Establish disease causation

 Provide basic information about what causes or

sustains disease in populations.

Guide healthcare policy and health planning.

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 Assist in the management and care of health and
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disease in individual

 Evaluations of preventive, diagnostic and therapeutic

programmes and technologies.

 Define standards and ranges for normal values of

biological and social measures.

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Basic Epidemiologic Assumptions
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The two basic assumptions in epidemiology are:

1. Human disease does not occur at random

2. Human disease has causal and preventive factors

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 Basic epidemiological assumptions….
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Human disease does not occur at random:

There are patterns of occurrence in which some behavioral

and environmental factors (exposures) increase the risk of
acquiring/developing a particular disease among group of
individuals.

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 Basic epidemiological assumptions……
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Human disease has causal and preventive factors

 identified through systematic investigation of populations

or group of individuals within a population in different

places or at different times.

 Thus, identifying these factors creates opportunity for

prevention and control of diseases in Human population

either by eliminating the cause or introducing appropriate
treatment.
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Epidemiological Concepts of Disease Causation

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Learning objectives
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At the end of the session the students will be able to:

 Describe the concept of disease causation

 Explain Models of disease causation

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 Introduction
Cause of Disease 28

is an event, condition, characteristic or a combination of

these factors which plays an important role in producing
the disease.
Characteristics of a cause
1. Must precede the effect
2. Can be either host or environmental factors
e.g., conditions, actions of individuals, events, natural,
social or economic phenomena
3. Positive (presence of a causative exposure –smoking
for lung ca) or negative (lack of a preventive exposure-
immunization for measles)

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 Classification of Causes of Disease

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1. Primary Cause / Necessary Cause: A factor that
must be present for the disease to occur. If it is
absent, the disease will not develop.

 In infectious diseases, this is referred to as the

etiologic agent.

Example: Mycobacterium tuberculosis is the primary

cause (etiologic agent) of pulmonary TB.

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2. Sufficient Cause: A set of conditions that inevitably

produce a disease when present.

 A disease may have multiple sufficient causes.

Eg. Rabies virus is a sufficient cause of clinical rabies.

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Risk Factors :
Increases the likelihood of31disease, but are not the
necessary causes.
Predisposing Factors: Make individuals more
susceptible to disease (Age, genetic susceptibility).
Contributing (Enabling) Factors: Help facilitate the
onset of disease by affecting exposure or resistance
E.g. Poor nutrition
Aggravating (Precipitating) Factors: Trigger or
worsen the disease condition. e.g stress

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 Risk factors can be categorized
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as:
 Modifiable: smoking, alcohol use, seat belt etc

 Non-Modifable: E.g Age, sex, genetics etc

 Etiology of Disease: is the sum total of all factors

(primary causes and risk factors) that contribute to the
occurrence of the disease.

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 Models of disease causation
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 Epidemiologists use various models to illustrate the

multifactorial nature of disease causation.

 While there are several disease causation models, some

of the most well-known include: Epidemiological

Triangle, Web of Causation, and Wheel Model.

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 Epidemiological Triangle
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 The most familiar disease model illustrates the
relationship among three key factors in the occurrence
of disease or injury: Agent, Environment, and Host.
 From the perspective of the Epidemiological Triangle,
the Host, Agent, and Environment must coexist
harmoniously for health to be maintained.
 Disease or injury occurs only when there is an altered
equilibrium or imbalance between these three factors.

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Agent: A factor whose presence, absence, excess, or
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deficiency is necessary for a particular disease or
injury to occur.
Host: Host factors influence an individual‟s exposure,
susceptibility, or response to a causative agent.
Examples include age, sex, race, socioeconomic
status, and behaviours (e.g., smoking, drug use,
lifestyle, eating habits).
Environment: All external factors, other than the
agent, that can influence health. These include social,
physical, and biological environments.

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Epidemiologic Triad (Balance Beam)
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 Web of Causation Model
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 This model explains that diseases result from a

complex interplay of multiple interrelated factors.

 It is particularly useful for understanding chronic

diseases, such as CVD, which are influenced by

factors like stress, diet, heredity, and physical activity

 This model emphasizes that diseases develop through

a chain of causation and can also be applied to injuries

and communicable diseases.
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Web of causation model

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 Wheel Model
 Core (Host): The center of the 39
wheel represents the host (human)
and their genetic make-up, which plays a central role in disease
susceptibility.
 The environment is divided into three sectors:
1. Biological: Includes pathogens (e.g., bacteria, viruses) and other
biological factors that contribute to disease.
2. Social: Factors such as culture, lifestyle, socioeconomic status,
and social interactions that impact health.
3. Physical: The physical environment, including pollution, climate,
and physical surroundings that affect health.
 The model stresses the unity and interaction of these factors in the
development of diseases.
 The relative size of the components depends on the disease.
e.g. For hereditary diseases, the genetic core is large.
For measles, the biological sector and host immunity are larger.

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Wheel Model
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Thank you!

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 SESSION 2: Concepts in Infectious Disease
Epidemiology
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Topics

 Chain of infection and mechanisms of transmission

 Transmission probability

 Infection timeline & natural history

 Levels of prevention

 Disease outcomes: Infectiousness, pathogenicity,

virulence
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 Learning Objectives
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 By the end of this session, students should be able to:

 Describe the chain of infection and transmission types.

 Understand transmission probability and apply it to real-world

settings.
 Outline the stages in the natural history of diseases.

 Classify preventive interventions by level.

 Differentiate key disease outcome terms: infectiousness,

pathogenicity, and virulence.

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