This post is REALLy making me not want to take my 2 year old and 1 year old to Disney this fall.
There's nothing about Disney World that makes people sick (except maybe the credit card bills).
Doctors know that people get sick more often when they travel than when they stay home.
WHY? It has to do with bacteria/viruses (which I'll call "germs") and our immune systems. Our immune systems learn to fight off germs by being exposed to them, by meeting them. When our immune system meets a germ for the first time, the germ will usually win the fight, and you will get sick.
But when our immune system and that same germ meet in battle again, our immune system will almost always win, because it now recognizes and knows how to fight and kill that germ. We breath in and eat millions of germs every day, but our immune systems easily fight them off and we never know about it.
But germs are smart little buggers. The "stomach bug" germs in Chicago are slightly different than the stomach bug germs in Seattle, which are slightly different from the germs in Philadephia, Cleveland, Los Angeles, etc.
So any time you travel away from home, you expose yourself to that new area's germs. Since those germs are slightly different from your "home" germs, your immune system is assaulted with germs it has difficulty fighting. This is even more common in a touristy area -- you not only have the native germs, but you have germs brought in from all over the country and the world by fellow travelers.
I'll give you another perspective. My wife and I grew up in Philadephia. We had the usual childhood stuff and were very healthy adults. Then she started medical school in town, and even though she was exposed to a lot more germs she didn't get sick any more often than before because her immune system spent a lifetime in the area, and it knew how to fight the local germs. All was good and well until she graduated and we moved to Cleveland where she started as a pediatrician. It's only 500 miles from Philly, but it's a whole new set of germs. We spent our first year getting sick during each new season (different germs are popular during different seasons), but after our first exposures to a one-year cycle of Ohio germs we remained pretty much sick-free for the next few years. Then we moved to the Atlantic City area, and we got sick all over again during that first year here, again be cause we were exposed to "new" germs that our immune systems didn't initially know how to fight.
So don't avoid Disney because you think your kids might get sick. There's no way to keep them germ-free. Just use common sense by washing hands as often as possible, using a hand sanitizer when washing isn't practical, and avoid putting hands to face.
HAVE FUN!