U.S. federal and state laws (and similar laws in many countries) prohibit employers from asking certain categories of questions that are not related to specific occupational qualifications of the job for which you're applying - illegal interview questions.
That is, employers cannot ask you personal interview questions that have nothing to do with the job you're being interviewed for.
It is illegal to ask questions about race, color, sex, religion, age (except to determine if you're old enough to do a job prohibited for youth), disability, marital status, national origin (except for government jobs that require a security clearance), arrest record (unless it resulted in a conviction), or ethnicity.
Individual U.S. states (and some other countries) also prohibit certain other personal questions, such as ones about sexual orientation.
As a jobseeker, you face the problem of how to handle illegal interview questions. If an interviewer asks an illegal question, it is usually out of ignorance, without malicious intent. The questions are still illegal, but unless the employer intends to use the information gained from the questions in a discriminatory way, the interviewer didn't commit a crime. Still, how are you going to answer?
If you get all up in arms and confront the interviewer or overtly refuse to answer the interview question, you will most likely eliminate yourself from the running for the job. And perhaps that's exactly what you want to do. That is, if the questions are blatantly insensitive or offensive you may not want to work for that company. If that's the case, there's nothing wrong with declining to answer and even leaving the job interview.
The more likely scenario is that despite the illegal question, you still want to be considered for the job. You may be able to skirt around a question, addressing the legal intent but not the illegal question. For example, if the interviewer asks about your health, you can say that you are fully qualified and able to perform all the duties of the job. If the employer asks if you are a U.S. citizen, you can say you meet all the requirements for working in the U.S.
Another option is to avoid the question and change the topic of conversation. Or, you can answer the question briefly, then move on. Even though it may be illegal to ask the question, it's not illegal to answer it. Just be careful not to provide any information that can harm your candidacy for the job.
You might want to do a little research before you start attending job interviews to make sure you understand what questions interviewers can and cannot ask. Here's a list of common questions at job interviews to help you.
And, as part of your preparation for an interview, practice responding to illegal interview questions. While you can't anticipate all the questions you might be asked, with practice you can get comfortable with how to answer illegal interview questions in general.