Honestly, I'd be careful saying something like that. Given that it's a disease involving the intestines, most people don't want to hear about it, and thus most people with the disease don't typically talk about it unless asked - and so a lot of people who have it don't mention it at all. It wasn't until I was diagnosed with the disease that I ever found out the people that I know who had it (and, in turn, my mother found out her friends who had it, or their sons, etc.).
Likewise, I don't generally tell people - it's not usually relevant to discussions, and no one really wants to hear about it. Chances are you know people who have it, but they've just never said so.
I can completely understand that - I guess I should have said that I am unaware of anyone with Crohn's; just as I am unaware of other conditions people might have. You are right that it would be foolish to assume you know someone based on their exterior alone and it's probably better to be sensitive to that possibility at all times.
Likewise, I don't generally tell people - it's not usually relevant to discussions, and no one really wants to hear about it. Chances are you know people who have it, but they've just never said so.