Incidence Versus Prevalence: What’s the Difference?

In 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).

Incidence, according to the Dictionary of Epidemiology, has to do with the number of new occurrences of an illness during a given period in a specified population. If I had framed my question to get information about the incidence of valley fever, I might have asked, “How many new cases of valley fever occurred in the past year in the Southwestern United States?” I would be asking about a specific number of new cases over a specific period of time.

So incidence statistics cover a period of time (How many new cases were there last year?) while prevalence statistics relate more to a specific point in time (What is the total number of cases—new and continuing cases—right now?).

Epidemiology is the study of the occurrence of disease or other problems that affect the health of people. Read more Epidemiology 101 on As Our Parents Age.