I was told by my gastro doc that IBS can cause Microscopic inflammation. Usually this inflammation is not visible to the naked eye. But who knows. I really do not think that they know. The doctors go by a list of criteria when it comes to diagnosing diseases. For instance, they say that with IBS you usually have to have at least three or more of the following( diarrheaha, or constipation or both alternating, Feeling relief after a BM, bloating, abdominal pain, heartburn, and gas.) Of course my GI told me that there is a Very wide variable when it comes to symptoms with IBS. Everyone is different and symptom can vary. With IBD, I think they can only diagnose it through a biopsy. That and if they do a scope and see tons of damage, then it is more likely to be IBD, not IBS. If you want my honest opinion, I think they just throw the Diagnosis IBS on all gut disorders until they can find out what the problem is or if it ever surfaces.