Describe the natural history of disease and the levels of prevention.

The Natural History of Disease

• It proceeds through two phases:
• The prepathogenesis phase — before the disease process begins in man, when the agent, host and environment are interacting but the individual is not yet affected. The disease has not started, but the conditions for it are being created
• The pathogenesis phase — from the entry of the agent onward, through subclinical pathological changes, the clinical horizon (the point at which signs and symptoms first appear), and then to recovery, disability, chronicity or death
• The clinical horizon is a key concept — it marks the boundary between what is detectable by the patient and clinician and what is not, and screening works by detecting disease below that horizon
• The incubation period (in infectious disease) and the latent period (in chronic disease) correspond to this presymptomatic interval
• The epidemiological triad describes the prepathogenesis phase — agent, host and environment in equilibrium, with disease resulting from a disturbance of that equilibrium
• Agent factors — biological (bacteria, viruses, parasites, fungi); nutrient (deficiency or excess); physical (heat, cold, radiation, noise, trauma); chemical, whether endogenous (urea, bilirubin) or exogenous (pollutants, allergens); mechanical; and the absence of a required factor
• Host factors — demographic (age, sex, ethnicity); biological (genetic, immunological, physiological); social and economic; and behavioural
• Environmental factors — physical (air, water, housing, climate); biological (vectors, reservoirs, other humans); and social (culture, customs, occupation, income)
• Multifactorial causation and the web of causation apply where no single agent is identifiable, as in most non-communicable disease; the web shows multiple interacting antecedents, and the disease can be prevented by cutting any of the strands
• The concept of risk factors follows — attributes associated with an increased probability of disease, which may be modifiable (smoking, blood pressure, diet) or non-modifiable (age, sex, genes)

The Levels of Prevention

• The modes OF intervention correspond to these levels — health promotion and specific protection (primary); early diagnosis and treatment (secondary); and disability limitation and rehabilitation (tertiary)
• The further from the onset OF disease the intervention acts, the cheaper and more effective IT IS, and this ordering is the argument for public health as a discipline
• The distinction between primordial and primary prevention is regularly examined: primordial prevents the risk factor from ever appearing; primary prevents the disease in someone who already has the risk factor
• To make it concrete — legislating against tobacco advertising so that children never start smoking is primordial; helping an existing smoker to quit is primary; screening a smoker for lung cancer is secondary; and rehabilitating a patient after lung resection is tertiary
• Rehabilitation has four components — medical (restoring function), vocational (restoring the capacity to earn), social (restoring family and social relationships) and psychological (restoring personal dignity and confidence)
• The high-risk versus population (mass) strategy is the other central idea in prevention, described by geoffrey rose
• The high-risk strategy identifies and treats individuals at greatest risk. It is appropriate to the individual, cost-effective, well motivated and has a favourable benefit-to-risk ratio — but is palliative and temporary, does not address the underlying cause, has a limited potential at population level, and depends on accurate screening
• The population strategy shifts the entire distribution of a risk factor in a favourable direction. It is radical, addresses the underlying causes, and has a large potential for the population — but offers small benefit to each individual, poor motivation for participants and clinicians, and a less favourable benefit-to-risk ratio
• The prevention paradox summarises the difficulty: "a preventive measure that brings large benefit to the population offers little to each participating individual" — which explains why population measures are unpopular and hard to sustain despite being effective
• The reason the population strategy works is that most cases arise from the large number of people at moderate risk, not from the small number at high risk — so treating only the high-risk group leaves most cases untouched
• In practice the two strategies are complementary and are used together, not as alternatives
• Primordial prevents the risk factor appearing and primary prevents disease in someone who already has it
• Advertising bans are primordial and quitting help is primary, which makes the distinction concrete rather than abstract
• Screening belongs to secondary prevention, detecting disease below the clinical horizon
• The clinical horizon is where symptoms begin, and screening works by looking beneath it
• Earlier intervention is cheaper and more effective, which is the whole argument for public health as a discipline
• Quaternary prevention protects from over-medicalisation, and is increasingly recognised as overdiagnosis grows
• Rehabilitation has four components — medical, vocational, social and psychological — and all four are needed
• Most cases come from the many at moderate risk, not the few at high risk, which is why population strategies work
• The prevention paradox explains unpopularity; a measure that helps the population greatly offers each person very little
• The two strategies are complementary, not alternatives, and are used together in practice
• Population strategies address underlying causes while high-risk strategies are palliative and temporary
• High-risk strategies motivate patients and doctors, which is their genuine practical advantage over mass approaches
• The prepathogenesis phase is before disease begins, when agent, host and environment are interacting but nobody is ill
• Incubation and latent periods are the same idea, applied to infectious and chronic disease respectively
• Agents may act by absence as well as presence, which is how deficiency diseases fit the model
• Risk factors are modifiable or not, and effort is concentrated on those that can be changed
• Specific protection includes immunisation and chemoprophylaxis, and is the most cost-effective primary measure
• Disability limitation comes before rehabilitation in the tertiary level, and the two are distinct activities
• Health promotion is not specific to any disease, unlike specific protection, which is directed at one condition
• Environmental factors include the social environment, not merely the physical and biological ones
• The triad works best for infectious disease and the web for non-communicable disease, which is why both are taught
• Disease may end in recovery, disability or death, and prevention aims to shift that outcome even after onset
• Host factors include behaviour, which is where lifestyle enters the triad and becomes modifiable
• Prevention is possible at every stage, which is the practical reason for learning the natural history at all