I work in a law firm, which means I have about 200 "bosses" (i.e., all the associates more senior than I and all the partners). I have pretty much told people I work with on an as-needed basis, but I definitely find it easier to be honest rather than not. For one thing, I DO cough, and about 3 months into my job I found myself faced with questions like "wow, hoe long have you had that cold anyway?" At that point, it was about 100% times easier to just calmly explain that the cough is chronic than it would have been to lie. I also hate lying about my CF - frankly I'm rather proud that I made where I am even despite extra challenges - so I'm pre-disposed to be honest about it.
As for explaining the seriousness or ramifications of the disease, that I choose not to do at work. I assume people google it, and then some come back and ask me questions (if they're comfortable with it) and others just leave well-enough alone. I show up for work virtually every day and do my job well, so I really think that's all my office needs to know, aside from the fact that I always tell in advance when I have medical appts. So my bosses know that I miss some work time due to illness or doctors, but they do not know, for example, that I'm considering transplant. That's a discussion for a later time.