As mentioned above, there's definitely a few different "types" of Crohn's, all with common characteristics but appearing in different parts of the GI tract. I believe the most common form, ileitus, has the disease isolated at the ileum (last part of small bowel). Disease higher up in the small bowel in the duodenum and jejunum is a lot less common, and from what I've read seems to have a distinct course; younger age of onset, requiring more frequent use of steroids and surgery (Tan & Allan, 1993). There's a few other types (Crohn's colitis for instance), but I don't know so much about them.
I don't think I've seen anything suggesting a distinct cause for each differing type of Crohn's. I imagine it has a large genetic component; with certain genes conferring a susceptibility to developing the abnormal immune reaction to
x environmental factor. Around 1 in 10 people with Crohn's have the disease both in the ileum and jejunum, but I've not read anything which suggests that there's a direct, distinct environmental cause (e.g: smoking, particular foodstuff, etc.) for this particular type of Crohn's disease. I guess I'm just one of the lucky 10%.
I didn't have any IBS-like symptoms before I got the first manifestation of Crohn's. Don't really find that diet helps ease symptoms much. Can help to control BM's and such, but the abdominal pain which occurs in a flare-up of symptoms doesn't ease off with any dietary changes. I assume that, like any illness, the severity can vary from person to person for a variety of reasons.
Might be completely wrong, though. My degree's in archaeology, so biomedical science isn't exactly my strong point.
