Meaningful of slides

1. Natural History of Diseases

2. Contents
• Introduction
• Natural history of disease
• Models of disease causation
• Factors of disease causation
• Patterns of Disease Occurrence

3. Introduction
• Knowledge of the natural history of disease ranks alongside
causal understanding in importance for disease prevention and
control.
• Natural history of disease is one of the major elements of
descriptive epidemiology.

4. Natural history of diseases
 Natural history of disease refers to the progress of a
disease process in an individual over time, in the
absence of intervention.
 The process begins with exposure to or accumulation
of factors capable of causing disease.
 Without medical intervention, the process ends with
 Recovery
 Disability
 Death.
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5. Natural history of disease
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6. ▶ Natural history of disease can be well
established by cohort study
▶ As these studies are costly and laborious,
understanding natural history of disease is
largely based on other epidemiological studies
such as cross sectional and retrospective
studies.
NATURAL HISTORY OF DISEASE

7. Natural history of diseases
 Consists of two phases :-
 PREPATHOGENESIS
The process in the environment
 PATHOGENESIS
 The process in the man
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8. Pre-pathogenesis 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
favor its interaction with the human host are already existing in
the environment.
 We all are in pre pathogenesis phase many of communicable
and non communicable diseases
 This situation is frequently referred to as “man in the midst of
disease” or “man exposed to the risk of disease”.
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9. Pathogenesis phase
 This phase begins with entry of the disease “agent” in the
susceptible human host.
 After the entry, agent multiplies and induces tissue and
physiological changes, the disease progresses through the
period of incubation and later through the period of early and
late pathogenesis.
 The final outcome of the disease may be recovery, disability or
death.
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10. NATURAL HISTORY OF DISEASE
DEA
TH
DISABILITY
RECOVERY
ENTRY OF
PATHOGEN
CLINICAL SIGN
OR SYMPTOM
DIAGNOSIS
PRE
PATHOGENESIS PATHOGENESIS
CLINICAL

11. ICEBERG CONCEPT OF DISEASE
DEATH
SEVERE
DISEASE
MILD ILLNESS
INFECTION WITHOUT
CLINICAL ILLNESS
EXPOSURE WITHOUT
INFECTION
CLINICAL
DISEASE
SUBCLINICAL
DISEASE

12. MODELS OF DISEASE CAUSATION
MIASMA THEORY
GERM THEORY OF DISEASE
EPIDEMIOLOGICAL TRIAD
BEING’S MODEL
WEB OF CAUSATION THEORY
WHEEL THEORY

13. MIASMA THEORY
n
 History of disease causation goes back
to Miasma theory
 It says “ diseases such as cholera,
chlamydia or black death caused by
Miasma, means bad air
 It was most accepted theory till 19th
century when it was replaced by Germ
theory of disease

14. Germ Theory of Disease
• Proposed by Robert Koch and Louis Pasteur.
• Every human disease is caused by a microbe or germ, which is
specific for that disease and one must be able to isolate the
microbe from the diseased human being.
ROBERT KOCH LOUIS PASTEUR

15. GERM THEORY
OF DISEASE
▶ Germ theory of disease was first
proposed in 1546
▶ Louis Pasteur proved that the diseases
are caused by organisms in 19th century.
▶ Supported by John Snow

16. Epidemiological Triad
16

17. Factors that may be associated with
increased risk of human disease
17

18. Agent characteristics
• Infectivity refers to the proportion of exposed persons who
become infected.
• Pathogenicity refers to the proportion of infected persons who
develop clinical disease.
• Virulence refers to the proportion of persons with clinical
disease who become severely ill or die.

19. Agent characteristics
• Hepatitis A virus in children has low pathogenicity and low
virulence, since many infected children remain asymptomatic and
few develop severe illness.
• In persons with good nutrition and health, measles virus has high
pathogenicity but low virulence, since almost all infected persons
develop the characteristic rash and illness but few develop the life-
threatening presentations of measles (pneumonia, encephalitis).
• In persons with poor nutrition and health, measles is a more virulent
disease, with mortality as high as 5-10%.
• Rabies virus is both highly pathogenic and virulent, since virtually
100% of all infected persons (who do not receive treatment)
progress to clinical disease and death.

20. Environment
• Physical environment
– Non living things and physical factors(air, water, soil, housing,
heat, light, etc)
• Biological environment
– Microbial agents, insects, animals, plants and man himself.
• Psychosocial environment
– Lifestyle, poverty, urbanization, community life, income,
education, stress etc.

21. ▶ As a result of advance in public health, communicable
diseases declined and rise of non communicable diseases
called modern diseases is seen.
▶ These diseases could not be explained by single cause
idea.
▶ It is now known that these diseases are due to multiple
factors which leads to formation of advanced model of
triangle of epidemiology.
EPIDEMIOLOGICAL TETRAD

22. Epidemiological Tetrad
• Agent
• Host
• Environment
• Time

23. The “BEINGS” Model of Disease
Causation
• This concept postulates that human disease and its consequences are caused
by a complex interplay of nine different factors –
– Biological factors innate in a human being,
– Behavioural factors concerned with individual lifestyles,
– Environmental factors as physical, chemical and biological aspects of
environment,
– Immunological factors,
– Nutritional factors,
– Genetic factors,
– Social factors,
– Spiritual factors and
– Services factors, related to the various aspects of health care services.

24. The Theory of “Web of Causation”
• The “epidemiological triad theory” was very effectively used
by Leavel and Clark in explaining the natural history of
disease and levels of prevention for obviating such departures
from the state of health.
• But it could not explain the causation of non communicable
diseases like IHD or road accidents.

25. Web of causation
• McMahon and Pugh forwarded the theory of “epidemiological
web of causation”,
hypercholesterolemia,
wherein the various factors (e.g.
smoking, hypertension) are like an
interacting web of a spider.
• Each factor has its own relative importance in causing
the final departure from the state of health, as well as
interacts with others, modifying the effect of each other.

26. Web of Causation applied to Myocardial Infarction

27. Example of Web of Causation
Susceptible Host Infection Tuberculosis
V
accination Genetic
Overcrowding
Malnutrition
Tissue Invasion and
Reaction
Exposure to
Mycobacterium

28. PHYSICAL ENVIRONMENT
SOCIAL ENVIRONMENT
BIOLOGICAL ENVIRONMENT
WHEEL THEORY OF
CAUSATION

29. Factors of Disease Causation
a) Predisposing factors are factors which create a state of susceptibility, so
that the host becomes vulnerable to the agent or to necessary cause, e.g.
age, sex, previous illness.
b) Enabling factors are those which assist in the development of (or in
recovery from) the disease; e.g. housing conditions, socio-economic
status.
c) Precipitating factors are those which are associated with immediate
exposure to the disease agent or onset of disease, e.g. drinking
contaminated water, close contact with a case of pulmonary TB.

30. Factors of Disease Causation
e) Reinforcing factors are those which aggravate an already existing
disease, e.g. malnutrition, repeated exposures
f) Risk factors : A risk factor is defined as a condition, quality or attribute,
the presence of which increases the chances of an individual to have,
develop or be adversely affected by a disease process. A risk factor is not
necessarily the cause of a disease but does increase the probability that a
person exposed to the factor may get the disease.

31. Exposure to
TB
Infection(10-30%)
No infection(70-90%)
Active TB(10% ill)
-5% develop TB within 2 years
-5% develop TB many years later
Latent TB (90% well)
-Never develop TB
-Not infectious
Treated
Untreated
Cured
50% die within 5 years
25% remain sick
25% recover

32. Spectrum of Disease
 The term “spectrum of disease” is a graphic
representation of variations in the manifestations of
disease.
 At the one end of disease spectrum are subclinical
infections which are not ordinarily identified, and
at the other end are fatal illnesses.
 In the middle of spectrum lie illnesses ranging in
severity from mild to severe.
 These different manifestations are the result of
individuals’ different states of immunity and
receptivity.
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33. Classification of diseases according to their
spectrum
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34. Class A: Inapparent infection
Examples: Tuberculosis, Polio, Hepatitis A, Meningitis, AIDS
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35. Class B: Classic cases
Examples: Measles, Chickenpox
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36. Class C: Severe or Fatal
infections
Examples: Rabies, Hemorrhagic fevers caused by Ebola
and Murberg viruses.
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