Paratuberculosis, caused by Mycobacterium avium subspecies paratuberculosis (M. paratuberculosis), is a chronic and incurable enteritis of ruminants with economic importance worldwide. The infectious agent is an acid-fast rod defined solely based on its mycobactin-dependent growth in vitro and the presence of insertion element IS900. The bacterium, which is difficult to culture primarily due to its extremely slow growth, occurs not only in cattle but also in other ruminant. In addition, it has been isolated from non-ruminant species. Despite its wide spectrum of potential hosts the contact between adult cattle and calves is the predominant route of infection within a herd as well as among herds. To interrupt this route of infection hygienic measures, primarily for the housing and feeding of calves, as well as diagnostic measures prior to trading of cattle are urgently required.