Incidence is the happening of new cases whereas prevalence includes both the new cases(incidence) and the old cases. for instance there were 50 TB cases from September - April 2012 from 600 at risk population but there were 3 new cases reported in May 2012. here the incident rate is 10/600= 0.017=1.7% whereas the prevalence rate at May 2012 is (50+10)/600= 0.1 = 10%. the prevalence rate at April is 50/600 = 0.083 = 8.3%. i think this may help you.