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Word Usage: Incidence and Prevalence

Incidence and prevalence are statistical measures of disease occurrence. Although these two words are closely related, they are not synonymous.

If you find these terms confusing, you are not alone. This word usage tip reviews the correct use of this troublesome word pair.


incidence (noun): the number of new cases of a specific disease in a given population over time
The incidence rate for asthma increased from 38 cases per 100,000 in 1990 to 40 cases per 100,000 in 1991.

prevalence (noun): the number of existing cases of a specific disease in a given population at any given time, regardless of when the disease developed
The prevalence of HIV infection in young adults aged 21 to 24 years was 0.1% in the United States in 2004.

Quick Tips: Prevalence and Incidence

Speedy pencil: quick grammar tips

  1. Incidence refers to new cases, whereas prevalence refers to existing cases.

  2. Period prevalence is the number of existing cases during a particular period of time, whereas point prevalence is the number of existing cases on a particular date.


Iverson C, Christiansen S, Flanagin A, et al. AMA Manual of Style: A Guide for Authors and Editors. 10th ed. New York, NY: Oxford University Press; 2007.

Council of Science Editors, Style Manual Committee. Scientific Style and Format: The CSE Manual for Authors, Editors, and Publishers 7th ed. Reston, VA: The Council; 2006.

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