29.1 The Physician as Health Care Provider I: Natural History of Disease, Disease Causation, Levels of Prevention and Primary Care Across the Clinical Rotations
Key Takeaways
In the Leavell and Clark model, the prepathogenesis period ends when the agent and host interact; the pathogenesis period runs from subclinical disease through clinical disease to recovery, disability or death.
Primary prevention (health promotion and specific protection) acts before disease; secondary prevention (early diagnosis and prompt treatment, including screening) shortens disease; tertiary prevention limits disability and rehabilitates.
Primordial prevention prevents the risk factors themselves, for example by taxing tobacco and sugary drinks or designing safe, walkable communities.
The epidemiologic triad of agent, host and environment explains infectious disease; the web of causation better fits non-communicable diseases with many interacting causes.
Bradford Hill criteria for causation include strength, consistency, specificity, temporality (the only essential one), biological gradient, plausibility, coherence, experiment and analogy.
Why this area matters
The TOS describes the physician as a health care provider in primary care during rotations in Medicine, Pediatrics, OB-Gyn and Surgery. Each case is approached through the same steps:
- the burden of disease: its age, sex and racial predilection, its local morbidity and mortality, the cost of illness to the family and its impact on national programs;
- clinical presentation and differential diagnosis;
- diagnosis;
- pharmacologic and non-pharmacologic management;
- health promotion and disease prevention, including a family health wellness plan.
Questions are usually clinical vignettes that ask which level of prevention an action represents, or what the best preventive step is for a patient or family.
The natural history of disease
Leavell and Clark describe two periods:
- Prepathogenesis: the agent, host and environment interact before disease begins. The person is healthy but exposed to risk factors.
- Pathogenesis:
- early, subclinical (asymptomatic) disease, which screening can detect;
- the clinical horizon, after which symptoms appear;
- clinical disease;
- the outcome: recovery, carrier state, chronic disease, disability or death.
Related concepts:
- Incubation period: for infections, from infection to symptoms.
- Latent period: for chronic diseases, from exposure to detectable disease.
- Carriers: infected people without symptoms who still transmit, for example typhoid carriers and hepatitis B carriers. They maintain the infection in the community.
- Spectrum of disease: many infections are mostly silent, as with polio, dengue and Japanese encephalitis. The "iceberg phenomenon" means clinical cases are only the visible tip.
The Epidemiologic Triad and Disease Causation
Infectious and non-communicable diseases develop through dynamic interactions among three classic elements forming the Epidemiologic Triad:
- Agent: The biological pathogen (bacterium, virus, parasite, prion), chemical toxin, physical force, or nutritional imbalance causing pathology.
- Host: The human organism susceptible to disease, influenced by intrinsic genetic vulnerability, age, sex, nutritional state, immunological competence, and behavioral patterns.
- Environment: Extrinsic factors that facilitate transmission or exposure, including physical climate, sanitary infrastructure, housing density, socioeconomic standing, and vector ecology.
- Vector / Vehicle: An intermediary living carrier (e.g., Aedes aegypti mosquito in dengue) or inanimate fomite/vehicle (e.g., contaminated water supply in cholera) transferring the agent from reservoir to susceptible host.
Other models and criteria of causation
- Web of causation: multiple interacting factors (diet, smoking, stress, genes, poverty) lead to disease. It fits non-communicable diseases such as coronary artery disease better than a single-agent model.
- The wheel model: the host, with its genetic core, at the center, surrounded by biological, social and physical environments.
- Necessary and sufficient causes:
- M. tuberculosis is necessary for TB but not sufficient: malnutrition, HIV, crowding and other component causes must also be present.
- Rothman's "causal pies" show how combinations of component causes complete a sufficient cause.
- Koch's postulates apply to infectious agents. The organism must be:
- present in every case;
- isolated in pure culture;
- able to reproduce the disease in a susceptible host;
- re-isolated from that host.
- They fail for organisms that cannot be cultured (M. leprae, T. pallidum) and for carriers.
- Bradford Hill criteria judge whether an association is causal:
| Criterion | Meaning |
|---|---|
| Strength | A large relative risk is less likely to be due to bias |
| Consistency | Repeated in different populations and studies |
| Specificity | One cause leads to one effect (weakest criterion) |
| Temporality | Cause precedes effect: the only essential criterion |
| Biological gradient | Dose-response relationship |
| Plausibility and coherence | Fits biological knowledge and the natural history |
| Experiment | Removing the exposure reduces disease |
| Analogy | Similar causes have similar effects |
- Social determinants of health (income, education, housing, water, sanitation, work, gender, access to care) are the "causes of the causes". They explain why poor Filipino families carry a higher burden of TB, malnutrition and diarrhea.
Levels of prevention
| Level | Aim and stage | Examples |
|---|---|---|
| Primordial | Prevent the emergence of risk factors in the population | Sin taxes on tobacco, alcohol and sugary drinks; smoke-free laws; safe water systems; urban planning for physical activity |
| Primary: health promotion | General measures that improve health | Nutrition education, breastfeeding promotion, exercise, housing, sanitation |
| Primary: specific protection | Protect against a specific disease before it occurs | Immunization, iron-folic acid in pregnancy, iodized salt, condoms, PrEP, chemoprophylaxis, helmets and seat belts, rabies PEP |
| Secondary | Early diagnosis and prompt treatment of existing but early disease | Screening (BP, glucose, Pap smear or VIA, newborn screening), contact tracing and treatment of TB, mass treatment in outbreaks |
| Tertiary: disability limitation | Limit complications of established disease | Glucose control and foot care in diabetes, thrombolysis in stroke, anti-TB treatment to prevent lung destruction |
| Tertiary: rehabilitation | Restore function and social roles | Stroke rehabilitation, prosthetics, vocational rehabilitation, leprosy self-care groups |
| Quaternary | Protect patients from overmedicalization and harm from unnecessary care | Avoiding unnecessary tests, antibiotics or polypharmacy |
Notes:
- The same intervention can belong to different levels depending on the target. Aspirin after a myocardial infarction is tertiary prevention. Treating hypertension to prevent stroke is primary prevention of stroke, and secondary prevention of hypertension.
- Rabies PEP is classed as specific protection because it is given after exposure but before disease.
Primary care across the rotations
| Rotation | Common primary care problems | Preventive priorities |
|---|---|---|
| Medicine | Hypertension, diabetes, dyslipidemia, TB, COPD and asthma, CKD | NCD risk screening (PhilPEN), smoking cessation, adult vaccines, TB case-finding |
| Pediatrics | Pneumonia, diarrhea, malnutrition, fever, skin infections | IMCI, immunization, growth monitoring, deworming, breastfeeding |
| OB-Gyn | Antenatal care, family planning, STIs, cervical screening | Eight ANC contacts, iron-folic acid, Td, facility delivery, postpartum FP |
| Surgery | Wounds, burns, injuries, hernias, breast lumps | Tetanus prophylaxis, injury prevention, early referral of suspicious masses |
The cost of illness to a family includes:
- direct medical costs (consultation, drugs, tests, hospitalization);
- direct non-medical costs (transport, food, lodging of companions);
- indirect costs (lost wages of the patient and caregivers, children kept out of school).
Out-of-pocket spending pushes families into poverty. Reducing it is a central aim of the Universal Health Care Act and PhilHealth benefits.
A barangay health team gives iron-folic acid tablets to all pregnant women, immunizes infants against measles, and promotes the use of iodized salt. Which level of prevention do these activities represent?
Primary prevention through specific protection
Secondary prevention through early diagnosis and prompt treatment
Tertiary prevention through disability limitation
Primordial prevention through risk-factor control
In a cohort study, heavy smokers had a 15 times higher risk of lung cancer than non-smokers, the risk rose with the number of cigarettes smoked, smoking always preceded the cancer, and similar findings appeared in many countries. Which Bradford Hill criterion must be met for any association to be causal?
Experiment: the association must be proven in a randomized trial
Analogy: a similar exposure must cause a similar disease
Specificity: the exposure must cause only one disease
Temporality: the exposure must precede the outcome
A 58-year-old man with diabetes for 15 years has a foot ulcer. The physician controls his glucose, treats the infection, teaches daily foot inspection and refers him for proper footwear to prevent amputation. Which level of prevention is this?
Primordial prevention
Tertiary prevention through disability limitation
Secondary prevention through screening
Primary prevention through health promotion
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