Never disclosed personal information at an interview!!!!! It is not important. I wouldn't say anything until you have an issue.
But also be aware, that merely having a doctor's note does not protect you from loosing your job. At this point, in the US, you don't really have any protections if you have a chronic condition, until you have been at your job for one year, and have worked 1250 hours in that year, to qualify for FMLA (Family Medical Leave Act).
ADA would not necessarily protect you from being fired for excessive abscences from work. ADA requires a REASONABLE ACCOMMODATION. This is a broad term but does not mean that an employer would need to accommodate someone missing work beyond the attendance policy or what they consider "excessive". Nor would it require an accommodation for tardiness. It could mean that you are placed closer to a bathroom or the flexiblity to work from home (if this is possible). I'm sure there are other accommodations, but it would depend on what you needed.
If you did disclose your condition at any point and were fired or not hired because of it, then you would have a case. But it would have to be because they fired you because you have an illness, not because you missed work. It doesn't really sound fair, but really if you think about it does make sense. If you can't work, you can't work. It is a burden for coworkers if everyone isn't at work as required.
I work in HR and recently had to terminate someone that has IBD. It was hard for me, and I felt great compassion for him (obviously!), but in his first two months of employment he was hosptialized twice and missed a total of about 3 weeks of work. It was unfortunate but we couldn't work with it. I did tell him I would work with him to find another position in the organization after he had gotten things a bit more under control. He emailed me last week and is feeling better so I am trying to place him in a position. He will be a great employee, I think, if he can get his symptoms under control. Really I think he tried too hard to come back to work before he was ready and it just snowballed for him.
Sorry if this sounds harsh but it is the reality out there.
Pancolitis since October 2004
Currently experiencing symptoms- blood and mild pain. Have started ANOTHER round of enemas.
My drugs of choice: Asacol 2-3 times a day (if I remember), Imuran, Canasa suppositories every other day, probiotics, fish oil, Iron, multi-vitamin
My other therapies of choice: Yoga, walking, my animals, and whining.