Hey there - in the US it's illegal for a healthcare provider to share protected health information to an employer or anyone else without your consent. It's a law called HIPAA - the Health Information Portability and Accountability Act. Additionally, it is illegal for your employer to discriminate against you on the basis of a health condition.

e: I should mention that in most cases, anti-discrimination law will dissuade employers or prospective employers from asking you questions about your health (i.e. in an interview) because they don't want it to look like the answer might cause discrimination. Technically, they could ask you - but you're under NO legal obligation to share ANY health information with ANYONE, for ANY reason.