Inflammatory bowel disease (IBD) are complex genetic disorders resulting from the complex interplay between several genetic and environmental risk factors. The number of IBD genes is currently unknown but it is expected to be equal or higher to 8. None of these genes are expected to be neither necessary nor sufficient for disease development. Among the candidate genes investigated to date, only CARD15/NOD2 has been definitively associated with Crohn Disease (CD) but not Ulcerative Colitis (UC). This gene explains about 20% of the genetic predisposition to CD. Because it is involved in the innate immunity, it allows to speculate that CD is the result of a defect in host-bacteria interaction. However, it is not known if CD is caused by specific bacteria or the gut flora as a whole.