Incidence: It is the number of times an event occurs. For  example the incidence of new businesses (opening of new ones) in Florida is 30,000 per year. And the incidence of businesses closing is 10,000 per year. Of the ones that closed, 1000 were related to software, 4,000 were related to selling of materials and 5,000 were related to service industry.

Prevalence: This is the number of an occurring event that has started but not finished. Therefore the prevalence is equivalent to the number of  businesses that exist. This is equal to the number of new businesses that survived plus the businesses that survived from the prior years. In this case it will be (30,000 - 10,000) plus whatever survived from the previous years.