What Exactly Do Incidence and Prevalence Mean?

The terms "incidence" and "prevalence" refer to the number of people who have a particular medical condition. "Incidence" means the number of people who are newly diagnosed with a condition, while "prevalence" of that condition includes newly diagnosed people, plus people who were diagnosed in the past, and, if the information is obtainable, people who haven't been diagnosed.

These terms are confusing, and some people use them interchangeably even though they do not mean the same thing, and are not interchangeable. Read on for more information on what they mean and how they're used in medical research.

Incidence and Prevalence Come From Epidemiology

Both incidence and prevalence are words used in the field of epidemiology. Epidemiology is a branch of medicine that looks at how many people have a particular disease, what the risks are of contracting a disease, what happens to people who get that disease, and ultimately, how to prevent disease from occurring.

One more term from epidemiology you should know is population. Epidemiologists study specific populations of people. For example, they might study adults who live in the U.S. as a specific population.

Populations can be broad (for example, all the children in China) or more specific (all elderly people of Asian descent living in New York City). Defining specific populations allows epidemiologists to determine what factors contribute to their disease risk, and how those risk factors might differ from risk factors for other populations.

Incidence vs. Prevalence

Here are some specific examples that might help you to get a handle on the differing meanings of incidence and prevalence.

If, for example, we say the incidence of celiac disease in active military personnel in the U.S. was 6.5 in 100,000 in 2008, that means six-and-a-half active military members for every 100,000 active military members were diagnosed with celiac disease in 2008.

Incidence tells you how many people were diagnosed with a particular disease in a particular population of people. In that case, the disease studied was celiac disease, and the population studied was active U.S. military personnel. 

Prevalence, meanwhile, tells you how many people have a particular condition, regardless of whether they were just diagnosed, or even whether they've been diagnosed at all.

To stick with our celiac disease example, a 2017 estimate of celiac disease prevalence in the United States in people six years and older indicates that prevalence is 0.7%. That means about one in every 141 people ages six and older in the United States has celiac disease. That figure includes people who have not been diagnosed with the condition. According to a study from 2012, most people with celiac disease (about 83%) don't realize they have it.

A Word From Verywell

You can't assume the numbers in one study on incidence or prevalence will apply to another population of people.

For example, just because the prevalence of celiac disease in the United States is 0.7%, you can't assume the prevalence of celiac in other countries—for example, in Asian countries—is the same as it is in the U.S., since those populations have different genetics and follow different diets and lifestyles. 

In fact, the prevalence of celiac disease is low in many Asian countries, in part because the genes for celiac disease aren't as common in Asian populations. However, the prevalence of celiac disease in Europe is close to the prevalence in the U.S., since genetics and diet in those two populations of people are similar.