Research has shown that HIV-positive patients sometimes refuse to take precautionary measures against sexual transmission as well as to notify their sexual partner(s) of their status. Faced with such a situation, physicians find themselves in a dilemma as they are forced to choose between honouring their duty of professional confidentiality and protecting their patients’ partner(s). Recent advice from the Belgian Medical Council (BMC) for the first time accepts that physicians can exceptionally, and under certain conditions, invoke necessity to breach confidentiality and inform usual and occasional sexual partners of HIV-infected patients who refuse to take precautions against transmission. This article sketches the ethical evolution of this position from 1987 up to now in Belgium, and examines whether or not the BMC's opinion on the matter corresponds to the applicable substantive criminal law provisions and whether or not partner notification could also be defended for other sexually transmissible infections (STIs). A case study of the situation in Belgium illustrates how discussions about STIs can deeply influence the evolution of legal and ethical rules about medical confidentiality.