Incidence and prevalence are two commonly misused terms that can be confusing to many people. That being said, their meanings are very different so keeping them straight is important, as misuse can completely change the meaning of a sentence.
Incidence is a measure of something occurring during a specific time period. If you were to discuss the incidence of mortgage foreclosures for a set time period, you would be discussing the number of loans specifically foreclosed during that time.

Prevalence in a way is the accumulation of the incidences taking place over a period of time. In the context of foreclosures mentioned above if we discuss prevalence we are talking about the total number of mortgages foreclosed. So if we counted the incidence of foreclosures related to the financial crisis every month for the last 5 years we could accumulate all of those incidences to determine the prevalence of foreclosures.

In application, say we have a .1% incidence rate of foreclosure each month out of a population of 1M mortgages. After one month the prevalence of foreclosure would be 1,000 mortgages. After one year the prevalence would be 12,000 mortgages, or 1.2% of the population. The incidence rate has stayed the same the whole time at .1%, but over time our prevalence has grown as more and more foreclosures have occurred.