Medical confidentiality: What does ‘need to know’ really mean?

Q: “I am currently taking training on the ADA and I understand the overall guidance regarding medical confidentiality. What I find confusing is the definition of the ‘employer’ and what HR, managers and supervisors are allowed to know about employees' medical conditions. Often an employee discloses a condition to their manager directly. And how can the two parties engage in the interactive process without the employee letting the supervisor know what the issue is? I've heard the answer to this is, ‘It’s need to know.’ For example, an employee tells their manager they need to start their job later due to ‘problems’ they are having. Can the manager ask what the problems are and if they are medically related, or should they send the employee directly to me?” – Anonymous, New York