Asking if you are healthy is not, per se, illegal, although I agree it is walking the line.
There is a fine line between questions that are out and out illegal (of which there are very few) and using the answer to questions regarding protected characteristics to make employment decisions. For example, if an employer asks you what your national origin is, that is not technically illegal. It would be illegal if he used the answer to that question, to make the hiring decision. For that reason, a smart employer will not ask the question unless there is a bona fide occupational qualification for it, since if he doesn't ask the question, he can't be accused (rightly or wrongly) of using the information to make hiring decisions.
One of the very, very few questions that is outright illegal is, are you disabled? But the employer did not ask you that; he asked if you were healthy. And he did not make the hiring decision based on your answer, or lay you off because of it. Therefore, there would be very little point to your taking any kind of legal action because of it. You suffered no damages as a result of his asking the question - the suit would cost you more than you would get back in return.