I used to just say something along the lines of "well that is how this disease is but you can't see the damage that is being done inside". Or "there are lots of diseases where people don't look sick but actually are". That's nice and non-confrontational and isn't even making it all about us Crohnies but it does suggest that people get some empathy!!!
These days, having been through that much more with this disease, I like to be a bit more challenging with something like "well you can't see my insides" and then tell them exactly how they are in truth not pretty, I've had lots of strictures and a big nasty abscess, so there's still scar tissue, adhesions, everything that comes with that. And I can say with confidence that this is what my surgeon reports back to me when he's seen inside. And *adds emphasis for the doubters* he is the one who has actually had a good look at everything and is the expert on this disease (i.e. not the person who is happily telling me I look well). It has the advantage of being factual and isn't me talking about what others can perceive as vague (because they are I suppose to a degree subjective) things like pain and fatigue. Although I don't think there's anything wrong with telling people how much those things affect you!
Or I say "yes, it's ironic isn't it because I've had three life-saving surgeries and my doctors now talk to me about my life-expectancy". Which is all true - not something I dwell on at all or find easy to believe myself but I do feel, depending on the person and the situation, that it can be appropriate to say to really bring it home to people that this disease can be life-threatening and that it shouldn't be dismissed.
I used to really struggle with others perception of my illness. When I was first diagnosed I had a doctor who told me that Crohn's patients who joined patient groups to talk about their disease were "whingers" (he was needless to say not right about that!) and after that a doctor who despite my continued pain on meds said he wasn't interested in hearing about my symptoms because I wasn't "the sickest patient he had". So I'd look around the waiting rooms and I'd compare myself - was I as sick as this very ill looking patient? Or that very underweight patient? The weight issue certainly bothered me when I was falsely robust and healthy looking with steroid weight gain. When people told me about their friends or relatives with Crohn's who'd been hospitalized I felt they judged me when I answered their questions and said I'd never been in hospital. But we all have different kinds of Crohn's and it affects us all in such different ways and we cope in different ways. And we are influenced to cope in different ways. I had been in pain for as long as I could remember and that meant that I had learned to cope with quite a level of pain and didn't see the need to go to hopsital to get help even when it was very severe because I never had even before I was diagnosed - and because I felt I had doctors who were telling me that I had no right to get help. I didn't for a long time have any emergency situation like a complete bowel obstruction, perforation, or abscess that would have precipitated admission to hospital. But that didn't mean that I didn't have serious disease that was progressing largely untreated.
I say all this because I feel that the stigma of having an 'invisible disease' made it harder for me to ask for help and get treatment. It made me feel less worthy of treatment. And as a result I got sicker and am probably sicker now than if I had had good treatment back then.
Now, having had experiences that were all too visible to others - extreme malnutrition, a life-threateningly low BMI, an ng tube that I wore out with a backpack pumping my liquid diet at 50ml per hour, being pushed in a wheelchair when I was too weak to walk - and knowing that this disease can be serious debilitating and even life-threatening - and having experienced that, I don't have any hesitation about telling people just what it means to have Crohn's and how not invisible the effects are - even if some people are blind to it. But I wouldn't want anyone to feel they have to go through these experiences - or similar ones - to speak with that weight of authority. Crohn's is a serious, debilitating, potentially life-threatening condition, even now in 2015. If people have trouble dealing with that it's their issue but I won't shy away from telling them.
I really do hope that our combined voices and experiences can empower each other to ask for help when we need it and to stand up for each other by educating those who have little or no knowledge of IBD.:ghug:
I might try that if the right situation crops up......