Employers with at least 15 employees are required to keep employee health information confidential pursuant to the Americans With Disabilities Act (ADA). The ADA also limits the types of health information an employer can seek to obtain from an employee. However, there can be situations where the employer is allowed to ask very personal information if the information is related to a request for a reasonable accommodation under the ADA. Check out this question and answer on Avvo:
https://www.avvo.com/legal-answers/is-an-employer-allowed-to-ask-you-if-you-can-pin-p-2976173.html