Crohn's disease is a chronic inflammatory bowel disease associating flares and remission periods. Its etiology is unknown and there are no specific therapy. CD affects young patients and has a major impact on quality of life. There are few population-based studies and there are about 2.5 million affected patients in Europe and North America. From data from EPIMAD Registry the number of affected patients in France should be 200 000. The Crohn's disease pathogenesis is bad known; It coul be the results of the activation of the gastro-intestinal immune system toward gut microbiota in genetically susceptible hosts. In CD patients there is an important ecologic modification of the flora with an excess of Bacteroidetes and Proteobacteria and a decrease of anti inflammatory bacteria (Firmicutes). In ileum of CD patients a specific E Coli (adherent and invasive E Colo) is found in two thirds of cases.The presence of this AIEC seems to be associated to the variant NOD2 (results from our team in multiplex families).