Public Health and Preventive Medicine
Public Health and Preventive Medicine tests epidemiology, screening, outbreak investigation, immunisation, and health promotion. Expect incidence and prevalence, sensitivity and specificity, notifiable diseases, and prevention levels.
Core rules
- Incidence measures new cases; prevalence measures existing cases.
- Sensitivity detects disease; specificity excludes disease. Positive predictive value depends on prevalence.
- Primary prevention reduces exposure; secondary prevention detects early disease; tertiary prevention limits disability.
- Notifiable diseases must be reported promptly to public-health authorities.
- Outbreak investigation: verify diagnosis, define cases, calculate attack rates, identify source, and control.
Worked example
In a foodborne outbreak, 30 of 60 people who ate chicken became ill, and 5 of 40 who did not eat chicken became ill. The reasoning is: attack rate exposed = 30/60 = 50%; unexposed = 5/40 = 12.5%. The risk ratio is 4.0, suggesting chicken is the likely vehicle. Confirm with interviews and laboratory testing, then remove the source.
Traps that cost marks
- Confusing incidence with prevalence.
- Assuming a highly sensitive test is always highly predictive.
- Forgetting that PPV falls when prevalence is low.
- Delaying notification because laboratory confirmation is pending.
- Calling screening beneficial without evidence of improved outcomes.