Public Health and Epidemiology
Public Health and Epidemiology tests how disease frequency is measured, how prevention is layered, and how an outbreak is investigated and controlled.
Core rules
- The epidemiological triad: agent, host and environment. Disease results from their interaction, which is why sanitation, nutrition and immunity all change risk.
- Incidence counts new cases in a population at risk over a period; prevalence counts all existing cases at one point. Incidence measures risk; prevalence reflects burden.
- Attack rate in a food-borne outbreak = (number of people affected ÷ number of people exposed) × 100.
- Levels of prevention: primordial (remove risk factors in the whole population), primary (immunisation, safe water, health education before disease), secondary (screening and early treatment), tertiary (rehabilitation and limiting disability).
- Surveillance means continuous collection, analysis and feedback; certain diseases are notifiable so that action follows the report.
- Outbreak steps: verify the diagnosis, confirm the existence of an outbreak, define and find cases, describe by time, place and person, form a hypothesis, test it, control, and write the report.
Worked example
At a wedding, 60 people ate the same paneer dish and 45 developed diarrhoea within 8 hours. Attack rate = 45 ÷ 60 × 100 = 75 %. Reasoning: a high attack rate confined to people who ate one food, with a short incubation, points to a food-borne intoxication or contamination, so the immediate action is to stop serving that food, collect samples and notify the health authority; the denominator is the exposed group, not the whole village.
Traps that cost marks
- Reporting prevalence when asked for incidence.
- Using the total population instead of the exposed group in an attack rate.
- Calling vaccination tertiary prevention.
- Describing an outbreak without a case definition.