This document discusses key concepts in epidemiology including definitions of disease, illness, and sickness. It describes the spectrum and "iceberg phenomenon" of disease, where most cases remain undiagnosed. Theories of disease causation are outlined, from older theories to modern concepts like the epidemiological triad/tetrad involving agents, hosts, and environments over time. Examples of risk factors, risk groups, and models of causation like the web and wheel of causation are provided. The natural history of disease progression is also addressed. Tuberculosis is used as an example to illustrate the interplay between infectious agents, human hosts, and social/environmental determinants of disease.

1. Concept of disease
Dr. Rizwan S A, M.D.,
Assistant Professor,
Department of Community Medicine,
VMCH&RI, Madurai.
24.10.2014

2. Outline
• Definition of disease
– Distinction between disease, illness, sickness
• Spectrum of disease
• Iceberg phenomenon
• Theories of causation
• Natural history of disease
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3. Spectrum of disease
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4. Iceberg phenomenon – 1/2
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5. Iceberg phenomenon – 2/2
1 Diseased, diagnosed & controlled
2 Diagnosed, uncontrolled
3 Undiagnosed or wrongly
diagnosed disease
4 Risk factors for disease
5 Free of risk factors
Diagnosed
disease
Undiagnosed or
wrongly diagnosed disease
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6. Theories of causation
• Old theories
• Modern theories
– germ theory
– multifactorial causation – socioeconomic, cultural,
genetic, psychological
– epidemiological triad (tetrad)
– web of causation
– wheel of causation
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7. Epidemiological triad/tetrad – 1/4
Agent
EnvironmentHost
Time
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8. Epidemiological triad – 2/4
• Agents
– Biological
– Physical
– Chemical
– Mechanical
– Insufficiency or excess of factors, hormones,
nutrients, parts, structural defects, chromosomal
factors, immunological factors
– Social
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9. Epidemiological triad – 3/4
• Host
– Demographic
– Biological – genetic, biochemical, immunological,
physio functions
– Social and economic
– Lifestyle
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10. Epidemiological triad – 4/4
• Environment
– Physical,
– Biological,
– Psychosocial
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11. Risk factors and risk groups
• Risk factors
– Modifiable, non-modifiable
– Examples for CVD
• Risk groups
– Target groups
– Risk approach
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12. Web of causation
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13. Wheel of causation
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14. Natural history of disease
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15. Example - TB
• Agent – types, sources, communicability
• Host – age, sex, genetics, nutrition, immunity
• Environment – urban slum, rural,
overcrowding, poor ventilation
• Other social factors – ignorance, stigma,
health facilities, health habits, beliefs and
customs
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16. Thank You
sarizwan1986@outlook.com