It is very hard when your child asks you this. We have been there before. A couple of years ago, my girls thought that you outgrow CF, since they personally didn't know anyone who had CF that was an adult. They said that when they grew up to be mommies that they wouldn't have CF anymore. I explained that it wouldn't go away, and that many adults have CF as well, it's just that they didn't know the people.
Then I guess that worked for a while, and my youngest must had forgotten about it. Because about a month ago while in the car there was something being said about CF (although I don't remember what it was) and my little one said "Do you mean I will always have CF?!" and when I said "yes, unless a miracle happens, you will" I saw her just turn and stare at her older sister like-'Did you know this!!?'
I am active in fundraising for the CFF, so they see and hear me doing things all of the time for CF, and they are understanding that the reason is that it costs a lot of money to make CF go away, and that even if it doesn't, there will be medicines to help keep them healthy.
Just answer the questions as they pop up as well as you can. Only you know what your child can comprehend. And most likely, at that age, at simple answer will do the trick. As they get older, they will become more curious, and I just go with the flow.
Unfortunately my nephew has CF as well, and he is much more sick than what my girls are, and they do hear more 'bad' things about CF than what they should. But I try to keep my hearbreak away from them so as to not scare them. My girls really feed off of my emotions.
We are Christians, and if someone asks them about why they take enzymes, or whatever, they just reply that they have CF and that is the way God made them.
Good luck. I am sure that many people will have suggestions as to what may work for you. I know it hurts inside<img src="i/expressions/face-icon-small-sad.gif" border="0">