Legal Question Concerning Workers Rights Toward Medical Privacy

I am curious to know what rights do employees have when they are sick and on disablility and their manager asks them personal questions about their condition? How much do they need to know? Then after getting an ok to come back to work from your primary doctor and the company doctor, how much information does the managment need to know about your condition? Examples of questions that have been asked of me: What kind of medication am I on, What does it do? I have been asked direct questions about the condition and what it entails. How much do they have a right to know? What can I say when I feel uneasy about a question I feel is to personal?

Any info would be helpful! Thanks!