Prevalence  is the simplest descriptive measure for the frequency of a disease or condition; it simply describes the number of people who have a disease at a given point (or period) in time. Point prevalence describes the number of people with a condition at a single point in time, whereas period prevalence may use a span of time, such as the number of people with tuberculosis in India during 2002 or between 2000 and 2005. The result is a proportion and is often presented as a percentage. Prevalence does not count those who have previously had the disease, only those who have it at the time of the count or during the interval of interest. It also does not take into account when the disease occurred or what factors led to it.

           The equation is:

           For example, the prevalence of childhood obesity in North Carolina in 2007 was 33.5%. This means that an estimated 33.5% of children in North Carolina were overweight or obese at some point in 2007 (National Conference of State Legislatures).