I struggled with this same predicament recently as I went on my first professional interview. I talked a lot to my husband about if and how I should bring it up because it explains part of the reason why I'm not in school right now. Well, I decided I was just going to keep it simple and say "I got sick and took the semester off and realized in that time I wanted to switch my area of study." That way, it's vague and doesn't necessarily point to a scary disease, but still gets my point across. It's all about reframing your answers.
Well, I'm in the interview and I get asked the question about school and I start out, "Well, I got sick last semester..." but before I could finish my statement one of the interviewers says, "You don't have to tell us that!!" Haha!
And I just finished up my statement saying I took the semester off and ya da ya da. I think Ruth is right in what she said. At least in the interview process, the employer doesn't want to be put into a position where they could be accused of asking about your sickness (which they are not allowed to do). So, don't feel bad about not being more upfront about it.
Once hired, then that would be something you could discuss with your boss and possibly HR so they can accommodate your condition, as necessary.
Good luck with your interview!