Sign on to Medscape.com and read through the IBS Resource Center information. You'll find numerous medical papers (some 'readable,' some not so) on the why's and wherefore's of what a number of docs and researchers believe to be causative factors of IBS.
Some IBS is suggested to be post-infection, about a quarter could actually be celiac problems and other forms of IBS may be due to a nervous/immune system interaction that may have gone slightly screwy. There's a new test that's just come out that checks for a number of very real chemical modulators that may be elevated in those with IBS. This suggests that IBS is a very real entity - as most of us who have experienced IBS already know. It's just been difficult to get most of the medical profession to open their eyes.
No one knows what exactly causes it.
and to explain it would be to include its symptoms It is a set of symptoms that occurs in some people that causes...well you know the rest.
http://healthlink.mcw.edu/article/930590639.html
Quote:
The cause of IBS is not known, and as yet there is no cure. Doctors call it a functional disorder because there is no sign of disease when the colon is examined. IBS causes a great deal of discomfort and distress, but it does not cause permanent harm to the intestines and does not lead to intestinal bleeding of the bowel or to a serious disease such as cancer.
Above this quote first paragraph is the definition...which again..includes the symptoms.