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# Prevalence

- Measure

The share of a population that has the disease at a point or over a period, counting new and existing cases.

Updated Nov 5, 2026

## Key points

- Prevalence = existing cases / total population at that time (or period).
- Point: on one date; period: at any time during an interval (for example, a year).
- It rises with more new cases, longer duration (better survival without cure) and in-migration of cases.
- It falls with cure, rapid death, fewer new cases or out-migration of cases.
- It is the typical measure of a cross-sectional study.

On this page

- The basics Key points
- What it is
- Formula and reading
- Worked example
- What moves prevalence
- Markers
- How to read it
- Common pitfalls
- Memory aid

- Study Test yourself

- Connections Connections

## What it is

Prevalence counts everyone who has the disease, whenever it began. Point prevalence is measured on one date; period prevalence includes anyone who had the disease at some time during an interval. It sizes the burden of disease and helps plan services, but it is a poor tool for studying causes because it mixes how often disease starts with how long it lasts.

## Formula and reading

- Point prevalence = cases on the date / population on the date.
- Period prevalence = cases present at any time in the interval / average population over the interval.
- It is a proportion without units, usually given as a percentage or per 1,000.
- Reading: a high value can reflect high incidence or a disease that lasts a long time.

## Worked example

- Data: on 1 March all 5,000 adults of a municipality are examined; 350 have asthma.
- Point prevalence = 350 / 5,000 = 0.07, that is 7%.
- Between 1 March and the end of the following February, 50 new cases appear; the population stays at 5,000.
- Period prevalence (one year) = (350 + 50) / 5,000 = 400 / 5,000 = 0.08, that is 8%.
- Reading: 7 in 100 adults had asthma on that day; 8 in 100 had it at some time during the year.

## What moves prevalence

| Change | Effect on prevalence |
| --- | --- |
| More new cases (higher incidence) | Rises |
| Treatment that prolongs life without cure | Rises |
| Arrival of people with the disease | Rises |
| Faster cure | Falls |
| Rapid death of cases | Falls |
| Departure of people with the disease | Falls |

## Markers

- P
- %
- per 1,000

## How to read it

- Health authorities use prevalence to estimate how many visits, medicines or beds a population needs.
- For diagnostic tests, the prevalence in the group tested changes the predictive values.

## Common pitfalls

- Using prevalence as if it were a risk: it does not tell who became ill after an exposure.
- Reading rising prevalence as more transmission, when it may reflect better survival.

## Memory aid

Prevalence is a photo of the room: it counts everyone inside without asking when they came in.

## Test yourself

Think of the answer, then tap the question to see it.

## What separates point prevalence from period prevalence?

Point prevalence counts cases on one date; period prevalence counts anyone who had the disease at some time in an interval.

## A new treatment prevents deaths from a chronic disease but does not cure it. What happens to prevalence?

It rises, because patients live longer and remain cases for more time.

## Which study design typically measures prevalence?

The cross-sectional study, which looks at exposure and disease at the same moment.

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## Connections

## Related 4

- [Incidence](https://durgical.com/en/epidemiology/incidence/)
- [Prevalence, incidence and duration](https://durgical.com/en/epidemiology/prevalence-incidence-and-duration/)
- [Cross-sectional study](https://durgical.com/en/epidemiology/cross-sectional-study/)
- [How prevalence changes predictive values](https://durgical.com/en/epidemiology/how-prevalence-changes-predictive-values/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.

Sources: [Principles of Epidemiology | Lesson 3 - Section 2](https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section2.html) (Centers for Disease Control and Prevention, 2012); [Principles of Epidemiology | Lesson 3 - Section 1](https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section1.html) (Centers for Disease Control and Prevention, 2012).

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Representative model, for educational purposes only.

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