The difference between prevalence and incidence is that prevalence refers to the proportion of new and old cases of a disease in a given population at a given time. Incidence is the frequency with which a disease occurs in a population in which it is likely to occur at the time of observation. The numerator of the prevalence rate is the number of new and old cases of a disease in the population surveyed at a given time, regardless of the time of onset of those cases. The numerator of incidence is the number of new cases of a disease in the exposed population at a given period of time. Prevalence is the frequency of a disease obtained from cross-sectional surveys and is a measure of the presence or prevalence of a disease. Incidence is the frequency of the disease obtained from incidence reports or cohort studies and is a measure of the presence of the disease. Prevalence and incidence therefore measure different diseases.