That is a question an employer has no business asking in an interview. It's discriminatory and therefore illegal. I would run far, far away from a business that would ask you that.......heaven only knows what other illegal and/or unethical things they do to their employees. oop:
For what it's worth: don't disclose your illness to anyone at work unless you must ask for accommodations. If it doesn't affect you in the workplace, your boss and co-workers need never know.