If you find yourself on the receiving end of this question, the answer is both simple and complex. The simple answer is this: no one knows for sure. Despite years of investigation, the root causes of IBD have yet to be identified.
There is also a more complicated answer. Although we don't know exactly what causes IBD, there are certain environmental factors which are known to be associated. For example, IBD is largely a disease of the developed world, found more often in Europe and North America, as compared to places in the world which are less developed.
Similarly, Crohn's disease and ulcerative colitis are more common in urban areas than in rural areas. IBD is more frequently reported in northern than in southern climates. A high social, economical, educational or occupational status also increases the risk of IBD. Factors such as cigarette smoking, a family history and whether or not a person has had an appendectomy also tend to matter. Stress is also associated with IBD, but it is known more as a modifier of the disease, versus as a cause. Surprisingly, there is relatively little evidence for the role that diet plays in inducing IBD. Some foods are known to worsen the conditions of the disease, but in terms of a cause, there are no simple answers.
For the most part, humans don't like ambiguity. In fact, most of us crave an understanding of cause and effect, especially when it comes to a disease that impacts those we love. But while the cause of IBD is not clear, you can be assured of one thing-if you are living with IBD it's not your fault.
For more information on the causes of inflammatory bowel disease click here.