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‫الكبيسي‬ ‫القهار‬ ‫عبد‬ ‫وقار�Ĭ

Epidemiology
I Keep six honest serving-men: (They taught me all I knew)
Their names are
What and Where and When and How and Why and Who.
—Rudyard Kipling (1865–1936)

Epidemiological triad
host
environment
agent

Germ theory of disease
During the 19th and the early part of 20th century, this concept
Referred to as a one-to-one relationship between causal
agent and disease.
The disease model accordingly is :
Disease agent Man Disease
The germ theory of disease, though it was a revolutionary
concept, led many epidemiologists to
 take one-sided view of disease causation.
That is, they could not think beyond the germ theory of disease.
 It is now recognized that a disease is rarely caused by a
single agent alone, but rather depends upon a number
of factors which contribute to its occurrence.

 The germ theory of disease has many limitations.
For example, it is well-known, that not everyone exposed to TB
develops tuberculosis. The same exposure, however, in an
undernourished or otherwise susceptible person may result in
clinical disease.
 Therefore, modern medicine has moved away from the strict
adherence to the germ theory of disease
 There are other factors relating to the host and environment
which are equally important to determine whether or not
disease will occur in the exposed host.
 This demanded a broader concept of disease causation that
synthesized the basic factors of agent, host & environment
The model - agent, host and environment ,has been in
use for many years.
It helped epidemiologists to focus on different classes of factors,
especially with regard to infectious diseases

Epidemiologic triad
the Triangle has three corners (called vertices):
Agent, or microbe that causes the disease (the “what”
of the Triangle)
Host: or organism harboring the disease (the “who” of
the Triangle)
Environment: or those external factors that cause or
allow disease transmission (the “where” of the Triangle)
Agent
Host
Environment

Epidemiologic triad
It could be considered as a model or approach to analyze
health and disease
 Health is a balance between agent, host and environment
 Changes in any one of these three factors may result in
loss of health.
 Epidemiologist try to characterize the relationship among
agent, host and environment
 As epidemiology developed by study of infectious disease,
the triad can help to understand them
 However, it could be applied to all health problems e.g. NCDs

 It should be noted also that there is sometimes overlap
between these factors
The triad is not applied only to the causation
But to all the disease process including:
Management (diagnosis, treatment, availability
utilization of health services):
Accessibility
affordability
Prognosis

The Agent “What”
The agent is the cause of the disease.
When studying the epidemiology of most infectious
diseases, the agent is a microbe. (microorganism)
Disease causing microbes are bacteria, virus, fungi, and
protozoa
 Presence of an agent is not always sufficient to cause
the disease
Epidemiologic triad

A. The Agent “What” :
I. Biological:
is not limited to infectious diseases, microorganisms
contribute to cancer and other NCD (rheumatic heart, type 1
DM )
II. Chemical:
poisons
chronic exposures:
heavy metals e.g. lead and toxic materials e.g. asbestos
(could be occupational)
III. Physical
Radiation: accidents, occupations Injuries
Temperature high/low
Sun; Sunburn, dermatological cancer
Epidemiologic triad

B. The Host “Who”
Hosts are organisms, usually humans or animals, which
are exposed to and harbor a disease.
The host can be the organism as any animal, that gets sick,
as well (including insects and worms) carrier that may or
may not get sick.
Although the host may or may not know it has the disease
or have any outward signs of illness, the disease does take
lodging from the host.
 Different people may have different reactions to the
same agent.
For example, adults infected with the virus varicella (chickenpox) are
more likely than children to develop serious complications.

Characteristics of Person
1. Age
2. Sex
3. Marital status
4. Socioeconomic status
5. Religion
6. Occupation
7. Ethnic group (in addition to the previous factors)
8. Genetics (host or agent)
genetic diseases
Sickle cell, alkaptonuria
Multifactorial: genetic predisposition
The genes are partially responsible for the racial differences
Cancer Heart disease and NCDs

The Host “Who” :
9. Immunity
Natural Artificial
Passive Active: immunization
autoimmunity: auto-immune diseases
Rheumatoid arthritis Systemic lupus
Multiple sclerosis
10. Nutrition:
A. Breast feeding
 Breast fed infants specially exclusive breast fed are
 protected against diarrhea & other infections (ear, ARI)
 Better Growth and development
 Long acting effect: protection against DM and other
chronic disease
 Breast feeding benefits to mother e.g. protect mothers
against cancer breast

B. Nutrition pattern
Feeding pattern weaning
Eating pattern: diet
 This pattern includes
 Who will eat what and when
 And nutrition during sickness
 Good nutrition means good health
C. Nutritional deficiencies They are diseases and predispose
to other diseases and mortality
“Nutrition-related factors contribute to about 45% of deaths in
children under 5 years of age.”
Protein energy malnutrition
Trace elements
Iron anemia
Iodine deficiency
Both are affected by other
host and environment factors
and may result in illness

Anemia:
Anemia is a common disease that affects ~1.6 billion people
worldwide, especially infants and women.
WHO estimates that 42% of children less than 5 years of age and
40% of pregnant women worldwide are anaemic.
The prevalence of anaemia in children under five was highest in the African
Region, 60.2%
•In 2019, global anaemia prevalence was
• 29.9% in women of reproductive age (15-49 years),
• equivalent to over half a billion women
•29.6% in non-pregnant women of reproductive age,
• and 36.5% in pregnant women.

Stunting
(chronic protein energy malnutrition)
2021,the UNICEF-WHO-World Bank Group Joint Malnutrition
Estimates shows that
stunting prevalence has been declining since the year 2000
more than one in five,149.2 million children under 5 –were
stunted in 2020*,
22.0% of all children under 5 years were stunted in 2020
and 45.4 million under 5 years suffered from wasting
13.6 Million children <5 years old were affected by
sever wasting form in 2020

Obesity
WHO classifies weight of adults using body mass index (BMI):
BMI greater than or equal to 25, as overweight and
BMI greater than or equal to 30 as an obese
a survey 2021 estimated that around two billion adults WW are
currently overweight
The biggest health problems facing people from 30 different
countries a obesity was ranked fifth, behind COVID-19, cancer,
mental health, and stress.
13% of adults in the world are obese.
39% of adults in the world are overweight.
One-in-five children and adolescents, globally, are overweight
38.9 million children <5 years were overweight. globally In 2020
5.7% of all children < 5 were overweight in 2020
Approximately 6% of adolescents (10-19 years old)WW were obese (2016)
This statistic shows a forecast of the estimated overweight population share in the
World until 2025. It is projected overweight to reach 42% by 2025.

Government health reports indicate that about
40% of Jordanian adults are overweight and child obesity
stands at more than 50%. “Of course we are fat. Feb 19, 2017
11. Behaviour
a. Personal hygiene
Simple measures of personal hygiene as hand washing
and tooth brushing can prevent many health problems
b. Smoking: COPD, cardiovascular, cancer
c. Addiction and drug abuse (dependence)
Risk behaviour: accidents, HIV
d. Healthy life style: diet, exercise
e. Compliance to treatment

The Environment “Where”
 The environment is the favorable surroundings and
conditions external to the host that cause or allow the
disease to be transmitted.
 Some diseases agent live best in dirty water. Others
survive in human blood. Still others, like E. coli, thrive in
warm temperatures but are killed by high heat.
 Other environment factors include the season of the year
(in the U.S., the peak of the flu season is between
November and March, for example). :
 Biological environment:
Includes all the living organisms in the environment that
may be agent, vector, environment for their breeding and
transmission, food resources,

Microorganisms and vectors
Agricultural and poultry and livestock (source of food, & disease
transmission)
Fishery Forests
Medicinal plants and herbs
 Physical environment
Season and temperature (weather)
Rural versus urban
Urbanization: slums and squatters
City planning: factories and sources of pollution
Traffic and transportation
Housing conditions overcrowding
Water Sewage and waste disposal
Recreational areas

The Environment “Where” :
 Social environment and Culture:
Cultural values and beliefs affect or control many health related
behaviours
 family formation pattern (age at marriage, spacing, &family
size)
 Consanguinity:
35% (population and family health survey 2012)
28% (population and family health survey 2017)
genetic diseases
 Health seeking behaviour
 Nutritional pattern
 technology

The Environment “Where” :
 Disaster and crisis:
Natural: earthquakes, storms
Man made: armed conflicts
 Destroys the resources
 Affects the infrastructure
 Housing
 Health, transportation
 High mortality and morbidity
The mission of an epidemiologist is
 to break at least one of the sides of the Triangle,
 disrupting the connection between the environment,
the host, and the agent, and
 stopping the continuation of disease.

Health determinants
 Many factors combine together to affect the health of
individuals and communities.
 Whether people are healthy or not, is determined by
their circumstances and environment.
 To a large extent, factors such as where we live, the
state of our environment, genetics, our income and
education level, and our relationships with friends and
family
 all have considerable impacts on health,
 whereas the more commonly considered factors such
as access and use of health care services often have
less of an impact.

 The main determinants of health include:
•Income and social status.
•Employment and working conditions.
•Education and literacy.
•Childhood experiences.
•Physical environments.
•Social supports and coping skills.
•Healthy behaviours.
•Access to health services.
 Individuals are unlikely to be able to directly
control many of the determinants of health.

Risk factors
 A risk factor refers to an aspect of personal habits or an
environmental exposure, that is associated with an increased
probability of occurrence of a disease
 A risk factor is a characteristic, condition, or behaviour that
increases the likelihood of getting a disease or injury. Risk
factors are often presented individually, however in practice
they do not occur alone.
 They often coexist and interact with one another. For example,
physical inactivity will, over time, cause weight gain, high blood
pressure and high cholesterol levels. Together, these significantly
increase the chance of developing chronic heart diseases and
other health related problems.
 Ageing populations and longer life expectancy have led to an
increase in long-term chronic expensive-to-treat diseases and
disabilities.

 Health risk factors are attributes, characteristics or
exposures that increase the likelihood of a person
developing a disease or health disorder.
 Behavioural risk factors are those that individuals have the
most ability to modify.
 Biomedical risk factors are bodily states that are often
influenced by behavioural risk factors.
 Since risk factors can usually be modified*,
 intervening to alter them in a favourable direction can reduce
the probability of occurrence of disease.
 The impact of these interventions can be determined by
repeated measures using the same methods and definitions
 Not all risk factors are modifiable, therefore it is important to
distinguish between modifiable and unmodifiable or less liable
to modification
 In general, risk factors can be categorised into the following
groups:

In general, risk factors can be categorised into the following groups:
 Behavioural
 Physiological
 Demographic
 Environmental
 Genetic
 Behavioural risk factors
Behavioural risk factors usually relate to ‘actions’ that the
individual has chosen to take. They can therefore be eliminated
or reduced through lifestyle or behavioural choices. Examples include:
•smoking tobacco
•drinking too much alcohol
•nutritional choices
•physical inactivity
•spending too much time in the sun without proper protection
•not having certain vaccinations
 unprotected sex.

Physiological risk factors
Physiological risk factors are those relating to an individual’s body
or biology. They may be influenced by a combination of genetic,
lifestyle and other broad factors. Examples include:
 being overweight or obese
 high blood pressure
 high blood cholesterol
 high blood sugar (glucose).
Demographic risk factors
Demographic risk factors are those that relate to the overall
population. Examples include:
 age
 gender
 population subgroups, such as occupation, religion, or income.

Environmental risk factors
Environmental risk factors cover a wide range of topics such as
social, economic, cultural and political factors as well as physical,
chemical and biological factors. Examples include:
 access to clean water and
 sanitation risk in the workplace
 air pollution
 social settings.
Genetic risk factors
Genetic risk factors are based on an individual’s genes.
 Some diseases, such as cystic fibrosis and muscular dystrophy,
come entirely from an individual’s ‘genetic make-up’.
 Many other diseases, such as asthma or diabetes, reflect the
interaction between the genes of the individual and
environmental factors.
 Other diseases, like sickle cell anaemia, are more prevalent in

Measuring risk factors
Risk factors can include tobacco and alcohol use, diet,
physical activity, blood pressure and obesity
Since risk factors can be used to predict future disease,
their measurement at a population level is important, but
also challenging.
Tobacco use can be measured
 by self-reported exposure (yes/no),
 quantity of cigarettes smoked, or
 by biological markers (serum cotinine
However, different surveys use different methods,
often with different measurement techniques and criteria
for detecting a risk factor or clinical outcome (for example,
diabetes or hypertension).

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Epidemiological Tried 7th Nov. 2022.pptx

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