Prevalence refers to the number of people in a population having a particular condition at a specific time. It is defined as

Prevalence per population unit (x) =

(where x is a multiple of 100)

Note: If you want to know the prevalence per 1000 population, substitute x with 1000. For prevalence per 10,000 population substitute x with 10,000 and so on.

From the previous article on incidence, you would have determined a few differences in the calculation of prevalence (compared to incidence):

The numerator

does not specify new cases- just cases, so both new and old cases are included

the number of cases are at a specific time- not a period of time

The denominator

Is not limited to persons at risk of developing the disease- all persons in the population at the specified time are included regardless of risk of developing the disease

The population count is for a specified time, not period of time

Therefore, this measure does not consider the time when a person developed the disease of interest. All persons with disease- whether they developed the disease yesterday, a month ago, or ten years ago- are included in the numerator. Since duration of disease is disregarded here, prevalence does not constitute a measure of risk. Think of it this way: when we want to measure risk of developing a disease, we typically want to know ‘how many of those who could potentially develop disease, actually develop disease in a specified period of time?’

What prevalence calculates instead, is the burden of disease– the proportion of the population that has disease at a specified time.