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).