Concept of disease, causation psm easy language to the point last minute revision highlights
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PSM - Concept of Disease & Causation: Last Minute Revision 🔥
(Source: Park's Textbook of Preventive and Social Medicine)
1. HISTORICAL THEORIES OF DISEASE CAUSATION
| Theory | Key Idea |
|---|---|
| Supernatural theory | Disease = punishment by gods |
| Theory of humors | Imbalance of 4 humors (blood, phlegm, yellow bile, black bile) |
| Miasmatic theory | Bad air / foul smell causes disease |
| Contagion theory | Disease spreads by direct contact |
| Germ theory (Pasteur, Koch) | One microbe → one disease (one-to-one relationship) |
Turning point: Louis Pasteur (1822-1895) - germ theory
2. GERM THEORY OF DISEASE
- Disease agent → Man → Disease (one-to-one model)
- Limitation: Not everyone exposed to TB develops TB. Not everyone exposed to streptococci gets rheumatic fever.
- Modern medicine has moved away from strict germ theory - towards multifactorial causation.
3. EPIDEMIOLOGICAL TRIAD (Classic Model)
Three factors must interact for disease to occur:
Agent
/ \
Host ——————— Environment
- Agent = cause of disease (seed)
- Host = organism harbouring disease (soil)
- Environment = external surroundings allowing disease
- Time = added in modern triangle (incubation periods, duration)
Mission of epidemiology: Break ONE leg of the triangle → stop disease.
Agent Factors (PBNRP)
- Physical agents: heat, radiation, noise, trauma
- Biological agents: bacteria, viruses, fungi, parasites
- Nutritional agents: deficiencies (e.g., vitamin A, iodine)
- Chemical agents: tobacco smoke, solvents, dietary factors, poisons
- Social agents: poverty, alcohol/drug abuse, unhealthy lifestyle
Modern concept of "agent" is very broad - includes both living and non-living agents.
Host Factors (Intrinsic - "SOIL")
- Demographic: age, sex, ethnicity
- Biological: genetic factors, blood groups, cholesterol levels, immunity
- Social/Economic: socioeconomic status, education, occupation, stress, housing
- Lifestyle: nutrition, exercise, smoking, alcohol, behavioural patterns
Environmental Factors (Extrinsic)
External = "all that which is external to the individual human host, living and non-living, with which he is in constant interaction"
Three components:
| Type | Examples |
|---|---|
| Physical | Air, water, soil, housing, climate, heat, radiation, noise |
| Biological | Vectors, reservoirs, pathogens in environment |
| Psychosocial | Culture, customs, social habits, mental stress |
4. MULTIFACTORIAL CAUSATION
- Pettenkofer of Munich (1819-1901) - first proponent
- "Single cause idea" is an oversimplification
- Non-communicable diseases (CHD, cancer, diabetes) cannot be explained by germ theory
- Example - CHD risk factors: high fat diet + smoking + obesity + physical inactivity (all together)
- Even TB is not just due to tubercle bacilli - poverty, overcrowding, malnutrition also contribute
- One-to-one relationship (cause-disease) is untenable even for microbial diseases
Models of Multifactorial Causation:
a) Web of Causation (MacMahon)
- Multiple causes interweave like a web
- Best understood for CHD
b) Wheel Model
- Center = genetic core (host)
- Surrounding = environment (biological, social, physical)
- Disease results from interaction
c) "Sufficient Cause" and "Component Causes" (Rothman's Pie model)
- A sufficient cause = minimal set of conditions that inevitably produces disease
- Each sufficient cause has multiple component causes
- No single factor is sufficient alone
5. CAUSAL RELATIONSHIP - HOW TO ESTABLISH?
Koch's Postulates (for infectious diseases)
- Organism must be found in all cases of disease
- Organism must be isolated in pure culture
- Pure culture must produce disease when inoculated into susceptible animal
- Organism must be recovered from experimental animal and identified
Limitation of Koch's: Cannot apply to non-infectious diseases; doesn't explain why some exposed people don't get disease.
Bradford Hill's Criteria of Causation (9 criteria - must memorize!)
| # | Criterion | Simple Meaning |
|---|---|---|
| 1 | Strength of association | Strong RR/OR → more likely causal |
| 2 | Consistency | Same result in different studies/populations |
| 3 | Specificity | One cause → one specific disease |
| 4 | Temporality | Cause must PRECEDE effect (most important!) |
| 5 | Biological gradient | Dose-response - more exposure → more disease |
| 6 | Plausibility | Biologically makes sense |
| 7 | Coherence | Does not contradict known biology |
| 8 | Experiment | Removal of cause reduces disease |
| 9 | Analogy | Similar cause-effect relationships known |
Mnemonic: Strong Consistency Specifically Times Biological Gradient Plausibly Coheres Experimentally Analogously = SCTS BG PCEA
Temporality is the ONLY absolute criterion (cause must always come before effect).
6. SPECTRUM OF DISEASE
- Graphic representation of variations in disease manifestation
- One end: subclinical/inapparent infection
- Other end: fatal illness
- Middle: mild to moderate illness
- Best example: Leprosy (spectral concept)
- Exception: Rabies (almost always fatal - no spectrum)
- Spectrum in infectious diseases = "gradient of infection"
- Interrupting the sequence at any point prevents further disease progression
7. ICEBERG PHENOMENON
- Disease in community = iceberg
- Tip above water = clinical cases visible to physician
- Submerged portion = hidden mass:
- Latent cases
- Inapparent/subclinical cases
- Presymptomatic cases
- Undiagnosed cases
- Carriers
- Waterline = demarcation between apparent and inapparent
- Examples of large submerged iceberg: hypertension, diabetes, anaemia, malnutrition, mental illness
- Detecting the submerged part is a challenge to preventive medicine
8. NATURAL HISTORY OF DISEASE
Two periods:
- Pre-pathogenesis period - before disease in man; interaction of agent, host, environment
- Pathogenesis period - disease in man; from first pathological changes to recovery/death
Levels of Prevention aligned to natural history:
| Stage | Prevention | Action |
|---|---|---|
| Pre-pathogenesis | Primordial + Primary | Health promotion, specific protection |
| Early pathogenesis | Secondary | Early diagnosis + prompt treatment |
| Late pathogenesis | Tertiary | Disability limitation + rehabilitation |
9. QUICK EXAM BULLETS
- Supernatural theory → Germ theory → Epidemiological triad → Multifactorial causation (evolution of disease causation)
- Koch = infectious disease causation; Bradford Hill = epidemiological causation
- Rothman's Pie = component causes form a "sufficient cause" - remove one component = break the causal pie
- Pettenkofer = father of multifactorial causation concept
- "Agent" in modern sense = PBNCS (Physical, Biological, Nutritional, Chemical, Social)
- Host = SOIL; Agent = SEED
- Iceberg tip = clinical cases; body = subclinical/latent
- Spectrum of disease: subclinical ←→ fatal; leprosy is classic example
- Temporality = only MANDATORY Bradford Hill criterion
- Breaking one leg of the epidemiological triad = stops disease transmission
Park's Textbook of Preventive and Social Medicine, Chapter on Concept of Disease and Causation (pp. 44-50)
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