Incidence Versus Prevalence: What’s the Difference?

incidence prevalence2In June 2013 I listened to a PBS News Hour segment about valley fever. A fungus, Coccidioides  (often called simply “cocci”) lives in soil in the southwestern United States as well as in Central and South America, and it causes valley fever. In the U.S the cases occur primarily in California and Arizona deserts and parts of several other states.

When the news story finished up, I asked my husband a question. “What is the incidence of Valley Fever? Just how many cases are there this year?”

“Aha,” he said. “Like lots of other people you are confusing incidence and prevalence.” So what is the difference between these two epidemiological concepts?

The question that I was asking — how many cases of Valley Fever had occurred in total — was not about the incidence of the disease but the about prevalence. According to the authoritative A Dictionary of Epidemiology, prevalence is the number of instances (cases) of a disease over a given time in a population. So my inquiry was actually a prevalence question — because I was wondering how many cases (of valley fever) there were in a given population (in the deserts/states where the cases were occurring) at a time (when I just finished listening to the program).

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Epidemiology: What Is It and Why Should Adult Children Know About It?

It happens over and over again as I listen to the radio or read the news. I hear about an aging parent issue or a disease that is increasing in magnitude. Or sometime it’s a health issue that is affecting certain groups of people or a new bit of research that describes problems with an intervention — one that I thought was working well. Invariably these stories make me ask why? Sometimes I ask a more personal question, “If that seems to work for me, how come researchers say is isn’t effective?”

In just about every case, I answer my question by learning more about the study of epidemiology — a field that explores and collects data about how diseases specifically and health issues in general occur and affect people and in certain places. Epidemiology measures by some period of time. This short video from the Centers from Disease Control explains more.

Epidemiology can be difficult to understand, especially because people, including me, tend to personalize the issues. Here are just a few questions to illustrate this personalization.

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