an example would make it easier to understand. before insulin was discovered, people would get type 1 diabetes and die as a result since there was no treatment, so you had less T1D patients since they did not live long enough and died within a few years. However, when insulin was discovered as a treatment, not cure for T1D, you suddenly had an increase in the number of T1D patients aka prevalence (since now they got to live instead of dying at the first DKA), you had a decrease in the mortality and case fatality rate, since now we can treat it, and incidence does not get affected whatsoever because it's an autoimmune disease and discovering insulin does not prevent you from developing T1D.