Short answer
Incidence counts new cases arising during a period, often expressed relative to a population at risk. Prevalence counts or measures all existing cases at a point or over a period, regardless of when they began. Think “new arrivals” versus “everyone currently present.” 1 2
Incidence tracks new onset; prevalence describes the existing burden, including old and new cases.
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At a glance
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| Question | Incidence | Prevalence |
|---|---|---|
| Cases counted | New cases | Existing cases, old and new |
| Time framing | During a defined period | At a point or during a period |
| Useful question | How fast are cases appearing? | How many people have the condition? |
The table summarizes the cited definitions and the article’s stated scope. 1 2
What each thing is
Incidence. The occurrence of new cases during a defined period in a specified population. 1
Prevalence. The proportion or count of people with a condition at a specified time or over a specified period. 2
Key differences
A condition can have low incidence but high prevalence if people live with it for a long time. A short-lived illness can have many new cases yet a lower point prevalence than expected. Denominators, case definitions and observation windows must be stated before comparing figures. 1 2
How to tell them apart
Look at the numerator and time window. “New diagnoses this year” points toward incidence; “people living with the condition today” points toward point prevalence. The words are sometimes reported as rates or proportions, so inspect the precise metric rather than relying on a headline. 1 2
Where they overlap
Both describe a condition in a population and can use the same surveillance system. Incidence contributes to prevalence, along with disease duration, recovery, death and population change. 1 2
Edge cases
A newly detected case may have begun earlier, complicating incidence. Period prevalence is not identical to point prevalence. Screening changes can affect observed counts without the underlying biology changing. 1 2
Why the distinction exists
Public health needs separate measures of the flow of new cases and the stock of existing cases. Prevention planning and care capacity answer different questions. 1 2
Common misconceptions
Prevalence is not simply the number of new cases, and a high prevalence does not automatically mean rapid current spread. 1 2
Examples
Ten people receive a new diagnosis this month: that contributes to incidence. If 200 people in the town are living with the condition today, that describes point prevalence in the town. 1 2
Sources
Sources checked October 3, 2026.
- CDC National Center for Health Statistics — Incidence. Definition.
- CDC National Center for Health Statistics — Prevalence. Definition.