A measure of the number of new cases of infection or disease divided by the population “at risk” of getting the infection or disease over a specified period of time. Incidence should not be confused with “prevalence” – a measure of the total number of infected or diseased individuals at a specific point in time. Example: Loftus et al. (Gastroenterology 114:1161, 1998) reported that the incidence of Crohn’s disease in Olmstead County Minnesota was 5.8 per 100,000 persons / year. On January 1, 1991 the prevalence of Crohn’s disease in that county was 133 per 100,000. For chronic diseases prevalence is always much higher than the incidence. There are many studies reporting prevalence of paratuberculosis, primarily in cattle, in different states, countries or regions of the world. Incidence of paratuberculosis, however, is much harder to measure and is seldom reported.