This doesn't completely relate...but me and my sister both have CF, and I am one of 6 kids (4 of them w/o CF). Anytime one of them was sick, I eventually caught it, and so did my sister, but in my case, nothing really serious ever came out of it. To me, it's all dependent upon how good your immune system is. I never really tried anything to prevent from getting sick except the obvious-no kissing, drinking from the same glass, etc. Personally I never really worried about getting sick, and my mom didn't much either. And that might be because me and my sister have a milder form of CF.
Also, in my own opinion, not being exposed to germs is what makes us all sick these days. Do you realize how many kids have allergies these days? We are all just so germ-free and bacteria-free conscious that we're not exposed to any of them anymore. I personally feel that is a major cause for everyone's prone-ness (can I make that word up?) to getting sick.