Public Health
Public Health tests epidemiology measures, screening, immunization, maternal and child health, environmental health, and outbreak response. The exam expects population-level reasoning and correct use of rates.
Core rules
- Incidence: new cases per population at risk over time. Prevalence: all existing cases at a point or period.
- Attack rate: new cases divided by population at risk during an outbreak.
- Screening: the condition should be important, detectable early, treatable, and the test acceptable; follow-up must be available.
- Immunization: maintain cold chain, correct schedule, and catch-up plans; contraindications are specific, not general.
Worked example
At a school lunch, 30 of 50 students who ate salad become ill, compared with 5 of 40 who did not eat salad. Attack rate in exposed is 30/50 = 60%; in unexposed, 5/40 = 12.5%; relative risk is 4.8. This strongly suggests the salad as the source. Reasoning: attack rates compare exposed and unexposed groups and generate a hypothesis for investigation.
Traps
- Confusing incidence and prevalence.
- Calculating attack rate using the whole population rather than those at risk.
- Recommending screening without treatment or follow-up.
- Ignoring cold-chain failure during immunization.