Medical Information Privacy in the Workplace

I work at a small non-profit organization (40 employees) and recently advised my supervisor that I would be requiring some time off work to have an operation for kidney stones. Not seemingly a privacy sensitive procedure but I do not like my medical information provided publicly around my office. A couple of days after telling my supervisor about the surgery, people started stopping by my office and wishing me well on the surgery and asking questions about my condition. I found out that the VP of Human Resources, at our company, announced that I would be having surgery to the entire senior staff (about eight managers) at a meeting.

What are my rights regarding what medical information I am required to disclose to my employer and is my employer legally required NOT to disclose medical information about me to other managers and/or employees of the company.

Thanks