# Definition of Prevalence

I have been informed that the prevalence of chickenpox and smallpox in the general population are 10% and 0.1%, respectively. On the Wikipedia page it is written

Prevalence in epidemiology is the proportion of a particular population found to be affected by a medical condition (typically a disease or a risk factor such as smoking or seat-belt use).

Should I read these numbers as 10 out of 100 people at any given time has chickenpox, knowingly or unknowingly? Similarly, 1 out of 1000 people in the general population has smallpox?

To me these numbers sounds very high.

• Well, one of those numbers is definitely too high. The prevalence of smallpox is 0% and has been since 1980. – Carey Gregory Feb 1 '19 at 18:58
• @CareyGregory the point prevalence (and the incidence) of smallpox is 0, but the lifetime prevalence is nonzero for some populations. – De Novo Feb 1 '19 at 19:41
• @DeNovo Ah, okay, thanks. – Carey Gregory Feb 1 '19 at 22:40

## 1 Answer

There are two broad types of prevalence statistics: point prevalence and period prevalence. In both cases, more details need to be specified in order to fully describe the statistic.

Point prevalence is the proportion of people who have a disease at a given point in time. In other words, the total number of people in a given population who have a disease at a specific point in time divided by the number of people in that population. To be complete, it should include a case definition (how "have a disease" is defined), a population definition, and the particular point in time used.

Period prevalence is the proportion of people who have or have at some point had a disease over a period of time. Here, instead of the total number with a disease at any one time, you include anyone who had the disease at any point during the period. Depending on the period you choose, the population may change, in which case, you would typically use the population at the midpoint of the period.

A specific (and common) period to use for this statistic is "lifetime", which changes things slightly. Lifetime prevalence is typically reported as the proportion of people (alive now) who have had the disease at any point in their lifetime. The period in this case is not an absolute time period, but the lifetime of each person in the population.

You can learn more about this in the CDC's online text Principles of Epidemiology in Public Health Practice. Lesson 3 covers prevalence statistics.