The epidemiologic triad is a model for explaining the organism causing the disease and the conditions that allow it to reproduce and spread. Epidemiological triad model demanded a broader concept of disease causation that synthesized the basic concept of agent, host and environment. This model helped epidemiologist to focus on different classes of factors, especially with regard to infectious disease.

1. IDENTIFICATION DATA
Name of the Student Teacher –Miss. Jaice Mary Joy
Name of the supervisor – Mrs. Pratibha
Subject – Community Health Nursing
Unit – III
Topic – Epidemiological Triad
Group of students – B.Sc. Nursing Part II
Venue – GCON, Jodhpur
Date & Time – 06/12/21, 1:00 PM
Duration – 45 minutes
Method of teaching – Lecture cum discussion
A.V. Aids – PPT, Chart, Flash card, Handouts, Poster
Previous knowledge of the group – The group has some knowledge regarding Epidemiological triad.

2. General objective – At the end of teaching the group will be able to enhance their knowledge regarding epidemiological triad and that will help
them to practice skill in the clinical setting while dealing health problem.
Specific objective – At the end of session the group will be able to-
1. Introduce the topic
2. Define epidemiology and epidemiological triad.
3. List down the types of epidemiological triad.
4. Explain the traditional triangle of epidemiology.
5. Elaborate the advance model of epidemiology triad.
6. List down the component of epidemiological triad.
7. Enumerate the agent factor of in epidemiological triad.
8. Explain the host factor of epidemiological triad.
9. Elaborate the environmental factor of epidemiological triad.

3. S.
No
Specific
Objective
Time Content
Teaching Learning Activity
A.V. Aids Evaluation
Teaching
Activity
Learning
activity
1. Introduce the
topic and
yourself
2 min INTRODUCTION
Epidemiology is a greek word that mean is:
Epi means among;
Demos means people;
Logy means study.
The epidemiologic triad is a model for explaining
the organism causing the disease and the
conditions that allow it to reproduce and spread.
Epidemiological triad model demanded a broader
concept of disease causation that synthesized the
basic concept of agent, host and environment. This
model helped epidemiologist to focus on different
classes of factors, especially with regard to
infectious disease.
Self Introduction
Myself Jaice
Mary Joy Student
of M.Sc. (N)
Previous, as per
my academic
requirement I am
going to represent
my topic –
Active
Listening
PPT
2. Define
epidemiology &
epidemiological
triad
3 min DEFINITION
Epidemiology -The study of the distribution and
determinants of health and disease related states in
populations, and the application of this study to
control health problems.
-By John M. Last
Lecture cum
discussion
Active
Listening
PPT What is
epidemiolog
y and
epidemiologi
cal triad?

4. Epidemiologic Triad - The epidemiological triad
or triangle is the traditional model for infectious
disease. The triad consists of an external agent, a
susceptible host, and an environment that brings
the host and agent together.
3. List down the
types of
epidemiological
triad.
1 min TYPES OF EPIDEMIOLOGICAL TRIAD
Epidemiological triad model of disease causation
has two types-
1. The traditional triangle of epidemiology
2. Advanced model of the triangle of
epidemiology
Lecture cum
discussion
Active
Listening
PPT What are the
types of
epidemiologi
cal triad?
4. Explain the
traditional
triangle of
epidemiology.
13
min
TRADITIONAL TRIANGLE OF
EPIDEMIOLOGY
It is based on the communicable disease model
and is useful in showing the interaction and
interdependence of agent, host and environment,
and time as use in the investigation of disease and
epidemics.
Agent
 Cause of disease
 Infectious disease - bacteria, viruses, parasites,
fungi and molds.
 Non-infectious disease - disability, injury or
death, chemicals from dietary foods, tobacco
Lecture cum
discussion
Active
Listening
Poster
& PPT
What is the
traditional
model of
epidemiolog
y?

5. smoke, solvents, radiation or heat, nutritional
deficiencies, or other substances such as
poison.
 One or several agent may contribute to an
illness.
Host
 Can be organism, human or an animal
 Offers subsistence and lodging for a pathogen
and may or may not develop the disease.
 Determining factors - level of immunity,
genetic make-up, level of exposure, state of
health and overall fitness of the host and the
ability of the pathogen to accept the new
environment because some pathogens thrive
only under limited ideal conditions.
Environment
 Surroundings & external conditions that cause
or allow disease transmission
 Includes the biological, social, cultural and
physical aspects of the environment.
 Surroundings in which a pathogen lives and the
effect the surroundings have on it are a part of
the environment.
 Can be within a host or external to it in the
community.

6. Time
 Incubation periods, life expectancy of the host
or the pathogen, duration of course of illness.
 Includes severity of illness in relation to how
long a person is infected or until the condition
causes death or passes the threshold of danger
towards recovery.

7. Epidemiological triad applied for infectious
disease
A traditional model of infectious disease
causation, known as the Epidemiologic Triad is
depicted in Figure. The triad consists of an
external agent, a host and an environment in
which host and agent are brought together, causing
the disease to occur in the host. A vector, an
organism which transmits infection by conveying
the pathogen from one host to another without
causing disease itself, may be part of the infectious
process.
In the traditional epidemiologic triad model,
transmission occurs when the agent leaves
its reservoir or host through a portal of exit, is
conveyed by a mode of transmission to enter
through an appropriate portal of entry to infect
a susceptible host. Transmission may
be direct(direct contact host-to-host, droplet
spread from one host to another) or indirect (the
transfer of an infectious agent from a reservoir to a
susceptible host by suspended air particles,
inanimate objects (vehicles or fomites), or animate
intermediaries (vectors)).

8. Figure 2: Epidemiologic Triad of Disease
Causation (Historical)
Epidemiological triad that applied for non-
infectious disease -
Epidemiologic triad can be applied to a disease
that not infectious? For example, a smoking-
related disease (Figure 3). If smoking causes the
disease, those who manufacture, sell and distribute
cigarettes are vectors, bringing the disease-causing
agent to the susceptible host. Diagramming the
epidemiologic triad also indicates potential
interventions to reduce disease in the population.
In this example, clean indoor air legislation,
advertising potential harm from smoking or
establishing workplace smoking cessation

9. programs could change the environment and
reduce the exposure of host to agent. Conversely,
increased advertising from cigarette manufacturers
or increased numbers of vendors would increase
exposure of host to agent.
Fig. 3: Epidemiologic Triad Applied to Smoking-
related Disease
Thus, the traditional model of disease transmission
can be useful to identify areas of potential
intervention to reduce disease prevalence, whether
infectious or non-infectious.

10. 5. Elaborate the
advance model
of epidemiology
triad.
10
min
Advanced model of the triangle of epidemiology
/ multifactorial causation –
Pettenkofer of Munich (1819-1901) was an early
proponent of this concept. As a result of advance
in public health, chemotherapy, antibiotics and
vector control communicable disease began to
decline- only to be replaced by new types of
disease, so called “modern” disease of
civilization, eg. Lung cancer, coronary heart
disease, chronic bronchitis, mental illness etc. it is
known that such type of disease are due to
multiple factors. For example, coronary heart
disease and cancer due to excess of fat intake,
smoking, lack of physical exercise and obesity are
involved in the pathogenesis of coronary heart
disease.
This model
 includes all facets of the communicable
disease model, and relevant and useful with
regard to today’s disease, conditions, disorders,
defects, injuries and deaths;
 Reflects the causes of current illness and
conditions.
 Behavior, life style factors, environmental
causes, ecologic elements, physical factors and
Lecture cum
discussion
Active
Listening
Chart
PPT
Explain the
advance
model of
epidemiolog
y triad.

11. chronic diseases must also be taken into
account.
 The term agent includes causative factor, risk
factors and environmental exposure which
implies the need to identify multiple causes or
a etiological factors of disease, disability,
injury and death.
The multifactorial concept offers multiple
approaches for the prevention/control of disease.

12. 6. Enlist the
component of
epidemiological
triad.
1 min COMPONENT OF EPIDEMIOLOGICAL
TRIAD
The epidemiologic triad is made up of three parts:
agent, host and environment.
1. Agent
2. Host factors (intrinsic)
3. Environmental factors (extrinsic)
Lecture cum
discussion
Active
Listening
PPT Which are
component
of
epidemiologi
cal triad?
7 Discuss the
agent factor of in
epidemiological
triad.
5 min 1. AGENT –
The disease agent is defined as a substance, living
or non-living, or a force, tangible or intangible, the
excessive presence or relative lack of which may
initiate or perpetuate a disease process. The first
link in the chain of disease transmission is a
disease agent. Disease agents may be classified
broadly into the following groups :
i. Biological agents – These are living agents of
disease, such as viruses, fungi, bacteria,
protozoa and metazoan. These agents exhibit
certain “host-related” biological properties
such as:
a) Infectivity- ability of an infectious agent
to invade and multiply in a host.
b) Pathogenecity- ability to induce clinically
apparent illness.
Lecture cum
discussion
Active
Listening
Flash
Cards
What is the
agent factor
of in
epidemiologi
cal triad?

13. c) Virulence- defined as the proportion of
clinical cases resulting in severe clinical
manifestation
ii. Nutrient agent – These are proteins, fats,
carbohydrates, vitamins, minerals & water.
Any excess or deficiency of the intake of
nutritive elements may result in nutritional
disorders.
iii. Physical agent – Exposure to excessive heat,
cold, humidity, pressure, radiation, electricity,
sound etc. may result in illness.
iv. Chemical agent –
a) Endogenous: Chemicals may be produced
in the body as a result of derangement of
function, e.g. serum billirubin (jaundice),
calcium carbonate (Kidney stone) etc.
b) Exogenous : Agent arises outside of
human host, e.g. allergens, metals, fumes,
dust, gases, insecticides etc. These may be
acquired by inhalation, ingestion or
inoculation.
v. Mechanical agents – Exposure to chronic
friction and other mechanical forces may
result in crushing, tearing, sprains, dislocation
and even death.

14. vi. Social agent – Poverty, social isolation etc.
8 Describe the host
factor of
epidemiological
triad.
5 min 2. HOST FACTORS ( INTRINSIC ) -
In epidemiological terminology, the human host is
referred to as ‘soil’ and the disease agent as ‘seed’.
In some situations, host factors play a major role
in determining the outcome of an individual’s
exposure to infection (e.g. tuberculosis)
The host factor may be classified as :-
I. Demographic characteristic such as age, sex,
ethnicity.
II. Biological characteristics such as genetic
factor ; biochemical level of blood ( e.g.
cholesterol) ; blood group & enzymes;
cellular constituents of the blood;
immunological factors and physiological
function of different organ system of the
body (e.g. blood pressure)
III. Social and economic characteristics such as
socio-economic status, education,
occupation, stress, marital status, housing
etc.
IV. Lifestyle factor such as personality trait,
living habit, nutrition, physical exercise, use
of alcohol, drugs & smoking, behaviour etc.
Lecture cum
discussion
Active
Listening
PPT What is the
role of host
in the triad of
epidemiolog
y?

15. 9. Elaborate the
environmental
factor of
epidemiological
triad.
5 min 3. ENVIRONMENTAL FACTORS
(EXTRINSIC )
For human being environment is not limited, as it
normally is for plants and animal, for example, For
man, social and economic condition are more
important than the mean of annual temperature.
Thus the concept of environment is complex and
all embracing. The external or macro environment
is defined as “all that which is external to the
individual human host, living and non-living, and
with he is constant interaction”.
The environment of man has been divided into
three components- physical, biological and
psychosocial.
i. Physical environment –non-living things and
physical factors (e.g. air, water, soil, housing,
climate, geography, heat, light, noise, debrise,
radiation etc.) with which man is in constant
interaction.
ii. Biological environment – living things which
surround man, like microorganisms, insects,
rodents, animals and plants including man
himself. These are constantly working for
their survival, and in this process, some of
them act as disease producing agent,
Lecture cum
discussion
Active
Listening
PPT How is
environment
important in
the triad of
epidemiolog
y?

16. reservoirs of infection, intermediate host and
vectors of disease
iii. Psychosocial environment – It is defined as
“those factors affecting personal health, health
care & community well-being.
 Includes cultural values, customs, habits,
beliefs, attitudes, morals, religion,
education, lifestyles, community life,
health services, social and political
organization
 Favourable social environment can
improve health, provide opportunities for
man to achieve a sense of fulfilment, and
add to the quality of life; eg community
participation.
 Unfavourable environment like migration
& exposure to stressful situations such as
bereavement, desertion, loss of
employment, birth of a handicapped child
may produce feeling of anxiety, depression,
anger, frustration etc. accompanied by
physical symptoms such as headache,
palpitation and sweating.

17. SUMMARY
Today we have discussed epidemiological triad. It has two types – one is traditional model of epidemiology and another is advance model of
epidemiology. Traditional triangle of epidemiology is useful in showing the interaction between agent, host, environment and time as used in
investigation of disease and epidemics. And advanced model of triangle of epidemiology includes all facets of the communicable disease model
and make it more relevant with today’s disease conditions. It has three components; agent, host and environment. Developing the disease
condition is the result of interaction between these three components.
CONCLUSION
The epidemiologic triad or triangle is the traditional model for infectious disease. The triad consists of an external agent, a susceptible host, and
an environment that brings the host and agent together. The primary mission of epidemiological triad is to provide information those results in
breaking one of the legs of the triangle, thereby disrupting the connection among environment, host and agent, and stopping the outbreak. This
discussion will help us to enhance our knowledge regarding developing of disease condition and how it can be prevent from occurring.
ASSIGNMENT
Make a poster on Epidemiological Triad

18. BIBLIOGRAPHY
1. Park K., Text book of Preventive and Social Medicine, Banarsidas Bhanot Publishers, 23rd Edition, Jabalpur, 2015, Pg. No – 33-36
2. BT Basavanthappa, A text book of Community Health Nursing, Jaypee Brothers Medical publishers, 2nd edition, New Delhi,
2008, Pg. No – 635-636
3. Gulani KK, Community health nursing: principles and practices, Kumar Publishing House; 1st Edition, Delhi, 2008
4. https://www.cdc.gov/csels/dsepd/ss1978/lesson1/section8.html
5. https://online.stat.psu.edu/stat507/lesson/1/1.2
6. http://samples.jbpub.com/9780763766221/66221_ch01_5398.pdf