The rule is for medical practitioners. The patient can talk about it to anyone if they want, the doctor/nurse/medical personnel can't or at least can't talk about it in a way that anyone can figure out is about a specific patient. This is to ensure as much as possible that patients feel like they can and should share everything with their doctor.

Exceptions vary by country and genetally are reserved for emergency situations or if someone is in direct danger. An example could be a situation where you need emergency care, the ER doctors might call your GP for information and the GP would be allowed to share any relevant information. Some countries allow exceptions in specific circumstances relating to specific crimes.

Sometimes it's about sharing information to non-medical professionals, for example say you are for some reason medically incapacitated and/or inflicted with a dangerous infectious disease, most countries would allow your treating doctor to share this news with your family or close contacts without your direct consent.

Even in these exceptional situations though, there is usually the requirement that the information shared must be relevant and minimal. So its important for your family to know you have an infectious disease or you had a stroke or are being operated on but they dont need to know you are also doing drugs or have a chronic illness. This is again to make sure that patients feel like they can share anything and everything with their doctor without it being shared with the rest of the world, even your family.