Prevalence is the proportion of persons in a population who have a particular disease or attribute at a specified point in time or over a specified period of time.

The numerator for prevalence includes all persons during a specified interval or point in time, regardless of when the condition began.

For example, a visual examination survey of 2477 persons between the ages of 52 and 85 years showed that 310 had cataracts. The prevalence of the condition was