## Definition

Prevalence is a measure of the health status of a population for a given disease.
It is measured by the number of individuals suffering from a disease for a given population, grouping together new cases and already existing cases.
Prevalence is to be distinguished from the prevalence rate, which represents the proportion of the population suffering from a disease.
Prevalence can also be expressed by population segment, generally per 100.00 people.

## Types of prevalence measures

There are 3 types of prevalence:

• Instantaneous prevalence : the number of cases of a disease at a given time
• Prevalence over a given period : the number of cases of a disease over a given period (year, month, etc.)
• Lifetime prevalence : the number of cases of a disease during the lifetime

## Instant prevalence example

On a territory, 2,000 cases are recorded at a time t for a population of 200,000 inhabitants.

• The prevalence is 2,000 cases
• The prevalence rate is 1.00%: (2,000 / 200,000)
• Expressed per 100,000 inhabitants, the prevalence is 1,000 cases per 100,000 inhabitants: (2,000 / 200,000) x 100,000

## Example of prevalence over a given period

The point to be taken into account over a period is that the population is evolving over time. Over the period studied, you must then take the average population during your study period.

On a territory, 5,000 cases are recorded over a year for an average population of 205,472 inhabitants over this period.

• The prevalence is 5,000 cases
• The prevalence rate is 2.43%: (5.000 / 205.472)
• Expressed per 100,000 inhabitants, the prevalence is 2,433 cases per 100,000 inhabitants: (5,000 / 205,472) x 100,000