Suppose a doctor prescribes a family member (who lives in the same home) some test for an infectious disease, and then that family member tests positive. Is it a violation of medical ethics to fail to disclose this illness to other members of the household, given that the illness is highly contagious?
Whether it's ethical or not is a moot question because in the US it would be illegal under federal law to do so unless the doctor has the patient's written permission to inform the others.
The law that prohibits this is known as the Health Insurance Portability and Accountability Act of 1996 (HIPAA). Specifically, the Privacy Rule of HIPAA identifies Protected Health Information (PHI) as follows:
Protected Health Information. The Privacy Rule protects all "individually identifiable health information" held or transmitted by a covered entity or its business associate, in any form or media, whether electronic, paper, or oral. The Privacy Rule calls this information "protected health information (PHI)."
The specific restriction is as follows (same link, emphasis is mine):
Basic Principle. A major purpose of the Privacy Rule is to define and limit the circumstances in which an individual’s protected heath information may be used or disclosed by covered entities. A covered entity may not use or disclose protected health information, except either: (1) as the Privacy Rule permits or requires; or (2) as the individual who is the subject of the information (or the individual’s personal representative) authorizes in writing.
Nowhere in the Privacy Rule does it permit informing family or cohabitants about infectious diseases -- or any PHI at all -- without the patient's permission.
Failure of ethics for whom?
- If the contagious patient does not disclose the risk for her/his family (and fails to protect them otherwise), endangering their health is a clear unethical violation.
- For medical staff: at least depending on the jurisdiction, there are areas where all patient data is highly confidential unless the patients wants the medical staff to talk, with the exception of the patient directly endangering herself/himself or others. A classical examples is a patient in an altered mental state who cannot understand that driving quite is likely to kill himself or a child on the road, or a HIV positive patient who is not willing to inform and protect partners. This is always a balance of the thread to confidentiality vs. thread to life of the patient and third parties, and requires a real and direct danger that cannot be resolved otherwise. In these cases medical staff is allowed (and required) to prevent immanent danger, and if necessary break confidentiality - as much as really necessary.
In addition, there are common obligations to tell about serious infections (depending on the disease for clinical practitioneer and/or diagnostic lab, either anonymously or by name), to allow health care authorities to monitor outbreaks and possibly take countermeasures for the endangered public.
In cases of doubt and when time allows, fellow team members (e.g. your supervising professor, clinic attorney) or a medical ethics board my be involved in the decision.
Regarding your example: if the disease causes substantial harm and is likely to be transmitted, it would be unethical (and unlawful depending on the jurisdiction) to force the family to suffer.