Public Health, Epidemiology and Sanitation
This is the technical core of the post. It tests how you measure disease, investigate an outbreak, and interrupt transmission through water, excreta and waste control.
Core concepts
- Measures of disease: incidence (new cases over a period) versus prevalence (all existing cases at a point); attack rate in an outbreak; case fatality rate.
- Levels of occurrence: endemic, epidemic, pandemic, sporadic.
- Outbreak steps: verify the diagnosis, confirm the existence of an outbreak, define and find cases, describe by time-place-person, generate a hypothesis, test it, control, communicate.
- Sanitation: protected water supply, chlorination with residual free chlorine of about 0.2–0.5 mg/L at the tap, sanitary latrines, safe excreta disposal, solid waste collection and food hygiene.
- Disease prevention levels: primary (immunisation, safe water), secondary (early diagnosis, screening), tertiary (rehabilitation).
Worked example
300 people attended a wedding feast; 45 developed diarrhoea within 24 hours. What is the attack rate and what does it suggest? Reasoning: attack rate = new cases ÷ people at risk × 100 = 45 ÷ 300 × 100 = 15 percent. A single event, a short incubation and a common exposure point to a food- or water-borne outbreak. Next, compare attack rates among those who ate and did not eat each dish to identify the vehicle, and start control: stop the implicated food, treat cases, chlorinate the water source, and collect stool and food samples.
Traps that cost marks
- Using total population instead of those at risk as the denominator.
- Mixing incidence with prevalence in a single question.
- Calling a single case an epidemic, or an outbreak an endemic situation.
- Forgetting that chlorination needs contact time and measurable residual chlorine.