What is the difference between period prevalence and incidence rate? I am confused about the difference between period prevalence and incidence rate. Following are from Wikipedia:

The incidence rate is the number of new cases per population in a
  given time period.
Period prevalence is the proportion of the population with a given
  disease or condition over a specific period of time.

Thanks!
 A: Incidence refers to new cases of a disease, while prevalence refers to all existing cases.
The two are related (Prevalence = Incidence x Disease Duration), but aren't the same thing. For example, for an incurable disease like HIV, the incidence rate could be decreasing as prevention measures improve, but the period prevalence can still rise (due perhaps to increased survival rates).
A: a)prevalence is the amount or the number of cases they have in a specified time(in other word the total number of cases divided by the number of individuals).example,300 players have participated in Ethiopia volleyball premier league in 2014 season,in that season 180 injuries were registered.Therefore,the prevalence of injuries are 180/300=0.6 .
b)incidence is the frequency of cases in a specified period of time.example:-in the 2014 Ethiopian volleyball premier league players spend 12345hrs in one season during game and training,in that period 180 injuries were registered. therefore the incidence of injury will be 180/12345*1000= 14.58
