**5. Conclusion**

Correct interplay between gut microbiota and the host is essential for human health. Microbial balance is pivotal for host metabolic and immune functions as well as to prevent disease development. Disturbance in that balance generates dysbiosis making the host susceptible to certain diseases. Gut microbiota stimulates the immune system, and altered composition of this microbiota in early life can lead to an inadequately trained immune system that can overreact to commensal microbes and lead to inflammatory diseases. Recent research has provided striking findings supporting that the gut microbiome plays an important function in the etiopathogenesis of IBD.

The clinical and epidemiological evidences showed that the infectious pathogens have possible role in IBD progression, especially, *Mycobacterium avium paratuberculosis*, *C. difficile*, *E. coli*, and *C. concisus*. Also, some viruses such as cytomegalovirus, Epstein-Barr virus, and measles by different pathogenesis have been be associated with the higher IBD risk; however, *H. pylori* may reduce intestinal inflammation and protect against IBD.
