Doctor-patient confidentiality was a medical and legal concept used in many cultures, especially in the Federation.
It held that information regarding an individual's medical condition and any related communications are protected from being divulged by the Doctor to any third party without the patient's consent. This applied to any third party, but especially to police agencies or other forms of authority, such as Starfleet.
In Starfleet such confidentiality can be broken if a Doctor is ordered to do so, but only if the security of the starship or Federation is at risk. Such was the case in 2371 when Kathryn Janeway ordered The Doctor to violate this confidentiality with regards to Samantha Wildman. (VOY: "Fury")
In 2152, Malcolm Reed asked if Phlox would respect doctor-patient confidentiality prior to discussing his gastrointestinal distress. (ENT: "The Catwalk")