Prevalence
- Measure
The share of a population that has the disease at a point or over a period, counting new and existing cases.
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.
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.
Connections
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 (Centers for Disease Control and Prevention, 2012); Principles of Epidemiology | Lesson 3 - Section 1 (Centers for Disease Control and Prevention, 2012).
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Representative model, for educational purposes only.