Many cystics are able to get away with this to the point of nobody even knowing they have the disease. For some, this is preferable, whereas there are others who believe in being open about it, educating people and "spreading the word". Neither approach is right or wrong, it simply is a matter of personal preference; what feels most comfortable to the individual.
For most of my life I have belonged to the former camp, preferring to blend in with the crowd and not share the fact that I have cystic fibrosis. Until about 10 years ago I was able to pull this off fairly well, with only the occasional remark about my cough ("You gotta give up those cigarettes".... if people only knew!) as acknowledgment that there was anything amiss. I suppose it happens to us all, at some point, when it becomes more necessary to divulge our illness.