Incidence vs Prevalence

Where this shows up in the AKT

• Epidemiology and public-health questions.
• Screening and service or resource planning.
• The classic trap: a “risk of developing disease” question tempting you with a prevalence figure.

Mix these two up and you will answer a screening question with the wrong number. Sort them out and a whole slice of epidemiology becomes easy marks.

The two definitions — becoming vs being

Get the definitions clean and everything else follows from them.

Incidence — becoming

• Incidence = the rate of new cases over a period of time, among the people at risk.
• It captures the risk of developing the disease — an event happening over an interval.

Prevalence — being

• Prevalence = the proportion of people who have the disease at a single point in time — existing cases and new ones together.
• It captures the current burden — a snapshot, not a flow.

Worked example — two numbers, two meanings

A town of 10,000 people. Over one year, 200 are newly diagnosed with a condition. At any given moment, 800 are living with it.

• The incidence is 200 new cases per 10,000 per year.
• The prevalence is 800 in 10,000 — 8% — at a point in time.

Notice the link: prevalence ≈ incidence × average duration. Two hundred new cases a year, each lasting about four years, gives 200 × 4 = 800 people living with it.

The bath analogy

For the clinical intuition, think of a bath filling and draining.

• Incidence is the tap — new cases flowing in.
• The plughole is recovery or death — cases leaving.
• Prevalence is the water sitting in the bath at any moment.

A long-lasting disease such as type 2 diabetes or COPD has a high prevalence even when new diagnoses each year are modest — the bath fills up because little drains away. A short illness, or one that is quickly cured or quickly fatal, drains fast, so prevalence stays low even when incidence is high.

The bath model: incidence is the tap (new cases in), recovery and death drain cases out, and prevalence is the water in the bath — so prevalence ≈ incidence × duration.

The twist examiners love

• A treatment that helps people live longer with a chronic disease, without preventing a single new case…
• …raises prevalence — more water in the bath, same tap.
• So rising prevalence does not always mean rising incidence; sometimes it just means people are surviving longer.

The traps that cost marks

Lock it in

• Incidence: new cases over time — the risk of developing disease.
• Prevalence: everyone living with it now — the current burden.
• Linked: prevalence ≈ incidence × duration.
• The bath: tap (incidence), plughole (recovery or death), water level (prevalence).