Public Health: Prevention, Screening, and Systems
Public health questions ask you to think at the population level: how disease is prevented, detected early, and controlled. Understand levels of prevention, screening principles, immunization programs, and the organization of health services.
Concepts to master
- Levels of prevention: primordial, primary, secondary, tertiary; matching interventions to each.
- Screening criteria: important disease, detectable preclinical phase, acceptable test, effective treatment, cost-effectiveness.
- Sensitivity, specificity, positive predictive value, and how prevalence changes PPV.
- Herd immunity and vaccine schedules; cold chain and contraindications.
- Health promotion, social determinants, and the role of surveillance systems.
Traps
Many questions confuse screening with diagnosis or primary prevention with early detection. A common trap is assuming a highly sensitive test is always best; in low-prevalence populations, even good tests yield many false positives. Another is confusing incidence (new cases) with prevalence (all cases). Watch for questions that ask about the best intervention at a population level rather than for an individual.
Worked rule of thumb
When asked whether screening is justified, run the criteria: is the disease important? Is there a detectable preclinical phase? Is the test acceptable and valid? Does early treatment improve outcomes? If any answer is no, screening is not justified. For test interpretation, remember PPV rises with prevalence and specificity.