Man, you don't know how lucky I feel! I just got back from a 5 day stay at WDW and it was so hot that I constantly drank from water fountains ALL over the place... and I didn't get sick once! We've been back two days now and (knock on wood) still nothing!
Was it the best tasting water I've ever had? No. (The winner is still the water out of a water fall in the backwoods of the Daniel Boone National Forest in Kentucky). But it still hit the spot when it was 90 degrees out and humid.
As for the idea that using a Brita system will keep you from getting sick, it's been pointed out that those filtration systems will NOT filter out bacteria that cause water-borne illness.
As for the sulphur. Again, there's no connection between a body's reaction to sulfa based drugs and sulphur. Case in point... everyone is "allergic" to cyanide. A little tiny bit of the stuff and, boom... your dead. But the toxic stuff is made of one part carbon and one part nitrogen... that's it: CN. You draw lots of nitrogen into your lungs with every breath and also ingest lots of carbon when you eat toast that's a little burned, well done steak, etc. But just because cyanide is lethal, doesn't mean its components are. The same is true with sulfa drugs. It's the way the elements are bound in the compound that causes the allergy, not the mere presence of sulphur.
So if you're body has become bottled water dependant, or you have deep seated fears of people slobbering all over WDW water fountains, or your stomach turns at the taste of WDW water... then you'll get no arguments from me about lugging around your on personal water supplies or taste filtration systems.
Dittos for the hand sanitizers when soap and water isn't nearby.
But also keep in mind that you're in the parks with 10's of thousands of other people from all over the world. Often times, you're in tight pre-show areas for periods of time... all the while inside the breathing space of those people around you. Anyone for wearing those cute little face masks that were all the rage in Asia during the height of the of SARS outbreak???
Remember, sometimes "Illness Happens".