Do you have any chronic health conditions we should consider?

(If not, then simply answer “No.” The question is illegal, and the interviewer is asking for your opinion. The only time you should answer this question positively is if you do have a condition that will become evident in a preemployment physical, like diabetes. In that case, you might answer:)
Nothing that would interfere with my work performance. I have had diabetes for ____ years, but I control it very successfully and unobtrusively. It has never become an issue in my work; and, aside from my family, only my doctor knows. It isn’t a secret—it just has nothing to do with performing on the job.