The incidence of a disease is the rate at which new cases occur in a population during a specified interval, e.g. "10 per year" or "12 in 1982". Data over a reasonable length of time can be used to calculate an approximate incidence. It is frequently given with a denominator of 1000, 10000 or 100000.
The prevalence of a disease is the proportion of a population that are cases at a point in time.