2. Burhan Uddin,
Karachi
2
Objectives
At the completion of this unit, the learners will be
able to:
1. Define the host, agent and environment terms.
2. Identify the concept of epidemiology and diseases
with the help of Epidemiological Triad.
3. Identify Epidemiological approach in community
setting.
3. Natural History of Disease
Iceberg Theory of Disease
3
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Karachi
4. 4
Iceberg Model / Theory
• The iceberg model is a systems thinking
tool designed to help an individual or
group discover the
– patterns of behavior,
– supporting structures, and
– mental models that underlie a particular
event.
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Karachi
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Natural History of Disease
• The natural history of disease is the course a disease takes in
individual people from its pathological onset ("inception")
until its eventual resolution through complete recovery or
death.
• Natural history of disease refers to the progress of a disease
process in an individual over time, in the absence of
intervention.
• This is the continuous progression in an individual of the
disease from the moment of exposure to the causal agents.
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Karachi
7. 7
Natural History of Disease
• Reflect on the four major possible outcomes in an individual of
exposure to a causal agent.
– First, the exposure may have no visible effect.
– Second, there may be demonstrable damaging effect of the
exposure which may be repaired.
– Third, the effect may be an illness that is rapidly contained by
the body's defence mechanism.
– Finally, the illness may progress until it leads to continuing long
term problems, irreversible damage or death.
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Karachi
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Natural History of Disease
• The process begins with exposure.
• Without medical intervention, the process ends
with
– recovery,
– disability, or
– death
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Karachi
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Stages in Natural History of Disease
• The stages in the natural history of disease are:
– stage of susceptibility
– stage of subclinical disease
– stage of clinical disease
– stage of recovery, disability or death
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Karachi
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Stages of Natural History Disease
Stage of Susceptibility – Begins with appropriate exposure to
accumulation of factors sufficient for the disease process to begin in
susceptible host.
Stage of Sub clinical disease – not clinically apparent and disease
will not develop; pathological changes occur but individual unaware,
diagnosed by antibody response or culture. Best time for screening.
Stage of clinical disease – clinically apparent, this is the best time for
diagnosis.
Stage of Recovery, Disability or Death – Ultimately, the disease
process ends either in recovery, disability or death.
Burhan Uddin,
Karachi
14. Figure:
Causes begin to exert
their influence here
Disease and first
manifestationcan be
diagnosed here, e.g.. MI
Time
Infancy childhood adolescence adulthood old age
Full health
Death
Recurrence
and death
14
Burhan Uddin,
Karachi
15. Class Group Activity
• Take name of common
disease in Pakistan
• Relate that disease into
stages of Natural
History Disease
• Then share with whole
class
15
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Karachi
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Applications of Natural History
⚫Natural history is vital for disease prevention policies.
⚫It underlies secondary prevention based on
screening
⚫It provides a rationale for all health care.
⚫Purpose of health care, including medicine, is to
influence the natural history of disease by reducing
and delaying ill-health.
⚫When achieved through purposeful actions by
societies the collective endeavour is public health.
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Karachi
17. past
Present
future
Public health and medical action
Time
Infancy childhood adolescence adulthood old age
Full health
Death
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Karachi
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Population Pattern of Disease
(PPOD
confused with the changing pattern of disease
⚫ Natural history of disease should not be (but is)
in
populations.
⚫ The distribution of a disease across socio-economic
groups may change as it has for coronary heart
disease.
⚫ This is called as “Population pattern of disease”
⚫ Main measures of PPOD are the disease incidence
and prevalence.
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Karachi
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Disease Process
The aspects of disease process include
⚫Etiology
⚫ Pathogenesis
⚫ Morphologic changes
⚫ Clinical manifestations
⚫ Diagnosis
⚫Clinical course
Burhan Uddin,
Karachi
24. Epidemiologic Triangle
⚫A traditional model of infectious disease
causation, known as the Epidemiologic Triad
⚫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.
⚫Epidemiologic
Homeostasis
Host
Agent
Environment
Host
Environment Agent
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Karachi
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Concept of Epidemiology & Disease vs
Epidemiological Triad
Epidemics arise when host, agent, and environmental
factors are not in balance
Due to new agent
Due to change in existing agent (infectivity,
pathogenicity, virulence)
Due to change in number of susceptibilities in the
population
Due to environmental changes that affect transmission
of the agent or growth of the agent
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