2. INTRODUCTION
The term ‘disease’ literally means ‘without
ease.’
There have been many attempts to define
disease.
Distinction is also made between the words
disease, illness and sickness which are not
wholly synonymous.
4. Definitions 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 functions”—Webster.
• “A condition of the body or some part or an
organ of the body in which its functions are
disrupted or deranged”—Oxford English
Dictionary.
• “A maladjustment of the human organism to
the environment”—Ecological point of view.
5. CONCEPTS OF CAUSATION:-
Up to the time of Louis Pasteur, various concepts of
disease causation were in vogue.
• the supernatural theory of disease,
•the theory of humors,
• the concept of contagion,
•miasmatic theory of disease,
•the theory of spontaneous generation,
etc.
6. Germ theory of disease
This theory gained momentum in 19th century.
The emphasis had shifted from empirical
causes (e.g., bad air) to microbes as the sole
cause of disease. The disease model according
to this theory is:
Disease agent --------→ Man----------→ Disease
7. Epidemiological triad
The germ theory of disease has many limitations.
• This demanded a broader concept of disease causation that
synthesised the basic factors of agent, host and environment.
• Interaction of these three factors 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.
ENVIRONMENT
AGENT
HOST
8. MULTIFACTORIAL CAUSATION:
Pettenkofer of Munich (1819-1901) was an early
proponent of this concept.
But the “germ theory of disease” overshadowed the
multiple cause theory.
It is now known that diseases 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 causation, web of
causation etc.
9. WEB OF CAUSATION:
• The “web of causation” considers all the
predisposing factors of any type and their complex
interrelationship with each other.
• The web of causation does not imply that the
disease cannot be controlled unless all the multiple
causes or chains of causation or at least a number
of them are appropriately controlled or removed.
• In a multifactorial event, therefore, individual
factors are by no means all of equal weight.
10. Web of causation in MYOCARDIAL
INFARCTION
Changes in lifestyle Stress
Abundance of food lack of physical exercise emotional disturbances
Smoking Aging and other
Factors
Obesity Hypertension
Increased catacholamines thrombotic tendency
Hyperlipidaemia
Changes in walls of arteries
Coronary atherosclerosis
Coronary occlusion
Myocardial ischaemia
Myocardial infarction
11. NATURAL HISTORY OF DISEASE:
Disease results 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
established by cohort studies. What the
physician sees in the hospital is just an
‘episode’ in the natural history of disease.
12. Prepathogenesis phase.
This refers to the period preliminary to the onset of disease in
man. The disease agent has not yet entered man, but the
factors which favour 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 ENVIRONMENT. These factors are known as
epidemiological triad. Even though these factors are
present, disease process will not occur unless there is an
interaction.
13. Pathogenesis phase
This phase begins 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 the disease may be
recovery, disability or death.
14. Agent factors:
The first link in the chain of disease transmission is a disease agent.
“Agent” is defined as a substance, living or non-living, or a force, tangible or
intangible, the excess presence or relative lack of which may initiate the
disease process.
• 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 agents like excessive heat, cold, radiation etc.
• Chemical agents. This could be endogenous (serum bilirubin,
ketones, uric acid) or exogenous (dust, gases, insecticide).
• Mechanical agents like friction, mechanical forces like crushing,
tearing, sprain etc.
• Absence/insufficiency or excess of a factor necessary to health.
Congenital heart diseases, chromosomal defects,
immunological factors etc.
• Social agents like poverty, smoking, alcohol and drug abuse etc.
15. HOST FACTORS
Intrinsic factors
• Demographic
characteristics like age, sex,
and ethnicity.
• Biological characteristics
such as genetic factors,
blood groups, and
enzymes, etc.
• Socio-economic conditions
like occupation, stress,
marital status, use of
alcohol, living habits etc.
Extrinsic factors
• This includes all man’s
surroundings such as
air, water, food, housing
etc.
• The environment has
three components;
physical, biological, and
psychosocial
environment.
16. SPECTRUM OF DISEASE.
The term “spectrum of disease” is a
graphic representation of variations in
the manifestations of disease.
The sequence of events in the
spectrum of disease can be interrupted
by early diagnosis and treatment or by
preventive measures like immunization.
17. ICEBERG OF DISEASE.
A concept closely related to the spectrum of disease
is the concept of the iceberg phenomenon of
disease. According to this concept, disease in a
community may be compared with an iceberg.
