I personally believe (and, I'm not a doctor, have no medical training or qualifications to try to back up my beliefs) that folks can AND do have both. One of the benefits of getting an IBD diagnosis is that means the doctors have found active inflammation or markers of it that allow them a clinical diagnosis. People who have gone thru the 'IBS' mystery illness get my sympathy. Simply, without positive, indisputable signs that there is inflammation, many docs will fallback on an IBS diagnosis because they haven't a clue what is going on. And, in hearing/experiencing what a friend of mine goes thru with their IBS, I'll take my IBD anyday. Reason? Well, I've a 'documented' acknowledged illness, I am given choices of meds to treat it or surgery to try to repair it, AND, (although many doctors still opine that food plays no part in it) I have been able, thru trial and error, to compile a list of my 'trigger' foods, and thus avoid them. People with IBS (or at least the folks I know with it or talked to) don't have those things going for them. Doctors dismiss them, there are no meds (since the doctors don't know what they're dealing with), surgery isn't an option, and trigger foods... that is a totally hit or miss proposition because what triggered an attack one day is perfectly safe to eat the next. I believe that folks with this ailment have weird fluucuations in their intestinal flora. Why? I dunno. Stress, hormonal changes, diet... I think these wild shifts in internal biotics is the culprit. It doesn't consist of anything that can damage the GI tract lining to leave a telltale sign (like Crohns does), or raise inflammatory markers, but it sure wreaks havoc.
It may be overpopulations of candida, or it may be bugs like h.pylori or c.difficile. or it may be any combination of natural internal biotics that simply rage out of whack from time to time. I know of one person in particular who finally found some relief from IBS by taking doses of acidophilous AND probiotics. It changed their nearly constant battle with IBS to one that only rears its ugly head on the odd occasion now. Anyway, the point I originally intended to make is that... I believe that many of us with confirmed cases of IBD dismiss the thought of IBS... maybe its a kneejerk reaction to before we were diagnosed and IBS was suggested (you remember, "its all in your head").. but that ALL of the symptoms we experience ALL of the times we experience them MAY not all be IBD.... but IBD compounded by occasional bouts of IBS, caused not by the same boogey man (eeer, bug) that gave us IBD, but by simple fluucuations in our intestinal flora... Remember, before we ever got ill with IBD, those occasional bouts of diarhea or whatever? Just because we have confirmed cases of IBD doesn't mean we can't also have IBS. Same as catching a cold doesn't mean we can't catch the flu too, or whatever. So, maybe, just maybe,, if by some long shot I'm right about this, it makes a lot of sense to watch for candida... take vitamins, acidophilous, probiotics, prebiotics, etc., etc..