1.
CONTENT
1. Introduction
2. Concept of Disease
3. Concept of causation
4. Natural History of disease in man
5. Spectrum of disease
6. Iceberg of disease

2.
LEARNING OBJECTIVE
To explain basic models of disease causation.
To understand the causes of disease .
To understand the applicability of causal criteria as applied to
epidemiological studies
To learn the Natural history of disease .

3.
DEFINITION OF DISEASE
• “A condition in which body health is impaired , a departure
from state of health, an alteration of human body
interrupting the performance of vital function ” --
Webster.
• “ A condition of the body or some part of an organ of the
body in which its functions disrupted and deranged ” --
Oxford English Dictionary.
• “A maladjustment of the human organism to the
environment” -- Ecological point of view.

4.
Up to the time LOUIS PASTEUR, various Concept of
DISEASE CAUSATION were inVOGUE
The supernatural theory of disease.
The concept of contagion.
Miasmatic theory of disease.
The theory of spontaneous generation,
etc.

5.
GERMTHEORY OF DISEASE
This theory gained momentum in 19th century .
The emphasis had shifted from empirical causes
(e.g. Bad air ) to microbe as the sole cause of
disease.
The disease model according to these theory is:
DISEASE AGENT MAN DISEASE

6.
The germ theory of disease has many limitations.
These demanded a broader concept of disease causation that synthesized the basic
factors of agent, host and environment.
Interaction of these three factor are needed to initiate the disease in man. This
epidemiological triad not only determines the onset of disease but also the distribution
of disease in the community.
ENVIROMENT
AGENT HOST
TIME

7.
MULTIFACTORAIL CAUSATION
Pettenkofer of Munich (1819– 1901)was an early proponent of this concept.
But the “the germ theory of disease” over shadowed the multiple cause
theory.
It is now known that disease such as coronary heart disease and cancer are due
to multiple factors.
For example excess of fat intake, smoking ,lack of physical exercise and obesity
are all involved in the pathogenesis of coronary artery diseases.
Therefore new models of disease causation have been developed . E.g..,
multifactorial, web of causation etc.

8.
 This model of disease causation was suggested by macmohan and pugh in their
book : “Epidemiologic Principle and Methods”.
 The “ web of causation” considers all the predisposing factors of any type and
there complex interrelationship with each other.
 The web of causation does not imply that the disease cannot be control unless all
the multiple causes and chains of causation or at least number of them are
appropriately control or removed.
 In a multifactorial event ,there fore individual factors are by no means all of equal
weight.

9.
Web of
causation in
MYOCARDIAL
INFARCTION

10.
Disease result from a complex interaction between man, agent(cause),
and the environment.The term natural history of disease is a key
concept in epidemiology.
 Each disease has its own unique natural history which is not
necessarily the same in all individuals.
 The natural history of disease is best establish by cohort studies.
What the physician sees in the hospital is just an ‘episode’ in the
natural history of disease.
NATURAL HISTORY OF DISEASE

11.
PREPATHOGENESIS PHASE
• This refers to period preliminary to the onset of disease in man.The
disease agent has not yet entered man, but the factors which favors its
interaction with the human host are already existing in the
environment.This situation is frequently referred to as “man in the
midst of disease ” or “man exposed to the risk of disease ”.
• The causative factors of disease may be classified as AGENT,HOST and
ENVIROMENT. These factors and known as epidemiological trade. Even
though these factors are present , disease process will not occur unless
there is an interaction.

12.
PATHOGENESIS PHASE
These phase start with the entry of “agent” .
The disease agent multiplies and induces tissue and
physiological changes .Thus the disease process started
and progresses through early and late pathogenesis.
The final outcome of disease may be recovery, disability or
death.

13.
The first link in the chain of disease transmission is a disease agent.
“Agent” is defined as a substance, living nonliving , or a force ,
tangible or intangible ,the excess presence or relative lack of which
may initiate the disease process.
AGENT FACTORS

14.
Biological agents: E.g., bacteria, viruses, protozoa etc.
Nutrient agents E.g. PEM occurs due to deficiency of protein, anaemia
due to lack of iron, etc.
Physical agent like excessive heat, cold, radiation etc.
Chemical agent . this could be endogenous (serum bilirubin, ketones
,uric acid) or exogenous (dust ,gases, insecticide)

15.
Mechanical agents like friction, mechanical forces like crushing
,tearing ,sprain etc.
Absence /insufficiency or excess of a factor necessary to health.
Congenital heart disease , chromosomal defects, immunological
factors etc.
Social agents like poverty ,smoking, alcohol and drug abuse etc.

16.
INTRINSIC FACTORS
 DEMOGHRAPHIC CHARACTERSTICS like age, sex , and ethnicity.
 BIOLOGICAL CHARACTERSTICS such as genetic factors, blood groups,
and enzymes, etc.
 SOCIO-ECONOMIC CONDITIONS like occupation, stress, marital status
, use of alcohol, living habits etc.
HOST FACTORS

17.
EXTRINSIC FACTORS
This includes all mans surroundings such as air, water ,
food, housing, etc.
The environment has three components; physical ,
biological, and physical environment.
ENVIROMENTAL FACTORS

18.
Positive health
Better health
Freedom from sickness
Unrecognized sickness
Mild sickness
Sever sickness
Death
SPECTRUM OF DISEASE
The term “spectrum of disease” is a
graphic representation of variations in the
manifestations of disease.
The sequence of event in the spectrum of
disease can be interrupted by early
diagnosis and treatment or by preventive
measures like immunization.

19.
A Concept closely related to the spectrum of disease is the
concept of the ice burg phenomenon of disease.
According to this concept , disease in a community may
be compared with an ice burg.
The floating tip of the iceberg represents what the
physician sees in community.
The “water line” represent the demarcation between
apparent and inapparent disease.
The major submerged portion of ice corresponds to hidden
mass of unrecognized diseases such as latent cases,
inapparent, carriers, asymptomatic and undiagnosed cases
in the community, which are all responsible for the constant
prevalence of the disease in the community.
THE ICEBERG OF
DISEASE

20.
References
 www.who.int
 http://nhm.gov.in
 IAPSM’s textbook of community textbook by Dr AM Kadri.
 A.H. Suryakanta ,Textbook of community medicine.
 www.whoindia.org
 www.iapsm.org.in
 k. park,Textbook of preventive medicine and social medicine