Environmental factors play an important role in the disease manifestation, course and prognosis of inflammatory bowel disease. Observations on temporal trends and geographical distribution point at risk factors associated with a Western lifestyle. A large number of studies have been performed on various factors such as diet, smoking, and several infectious agents. Childhood exposures modifying immune responses in later life form a particularly interesting field. However, so far, only smoking in Crohn's disease, and smoking cessation in ulcerative colitis can be considered established as risk factors for the manifestation of the disease. Smoking is also associated with a poor prognosis in Crohn's disease. A strong negative association of appendectomy with ulcerative colitis has been very consistent across many studies; however, the implications of this finding are still obscure.
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