I was an employer for over 20 years, so I had to be very familiar with a LOT of federal and state employment law, as well as many other non-employment laws such as HIPAA. Some of those laws I learned the hard way.

In reference to this specific issue, I would direct you to start with the Equality Act and the Americans with Disabilities Act.

But you need to consult with legal counsel that specializes in those areas, because the statutes and revised codes are not enough. Case law and court precedent often distort, confuse, and otherwise obfuscate the plain text of the law. Worse, one court will rule very differently on the same set of circumstances and evidence than a different court within the same district or circuit.

Consult an Employment Law attorney in your area.

....oh, and as for advertising your personal medical information on a billboard, there’s this little thing called “tort law” that would remedy that.
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