It is not legal for them to ask, and you are not legally bound to tell.
As far as telling post-job, I think it only matters IF your work is affected. If the environment causes problems, if you are expected to do something that will aggravate seriously your condition.
I do not tell, and do not take days off. I am lucky to be in a profession that has great personal leeway. However, no matter how sick I am, I go to work. It is a point of pride with me.
Post Edited (Myself 09) : 9/24/2012 4:47:18 PM (GMT-6)