Crohn's disease is a 55% chance the other will too. The incidence of Crohn's disease is increasing in Europe and in newly industrialised countries. For example, in Brazil, there has been an annual increase of 11% in the incidence of Crohn's disease since 1990. Inflammatory bowel diseases were described by Giovanni Battista Morgagni (1682–1771) and by Scottish physician T. Kennedy Dalziel in 1913. "Ileitis terminalis" was first described by Polish surgeon Antoni Leśniowski in 1904, although it was not conclusively distinguished from intestinal tuberculosis. In Poland, it is still called Leśniowski-Crohn's disease (). Burrill Bernard Crohn, an American gastroenterologist at New York
Burrill Bernard Crohn that it was caused by the same pathogen, a bacterium called "Mycobacterium paratuberculosis" (MAP), responsible for the similar condition that afflicts cattle called Johne's disease. However, he was unable to isolate the pathogen—most likely because "M. paratuberculosis" sheds its cellular wall in humans and takes the form of a spheroplast, making it virtually undetectable under an optical microscope. This theory has resurfaced in recent years, and has been lent more credence with the arrival of more sophisticated methods of identifying MAP bacteria. Crohn noted that it can be difficult for physicians to correctly differentiate neuroses from diseases of the organs.