The sentence "One cannot NOT communicate" is a double negative which makes it a positive saying. The meaning is like saying that because you can't not say it, you MUST say it. If you are in health care, the needs of the patient come first and they must be given the correct information. Sometimes the easier way is to keep bad information from the patient for a short time to give them time to adjust to the new diagnosis; however, in my opinion, that is bad medical practice as patients need information to make informed decisions. Therefore, even if you wish to not say it, you must communicate with your patient. So, a health care professional cannot avoid communicating or cannot NOT communicate what a patient needs to know. In other words, "you cannot not communicate" means that you must communicate; you have no choice, not communicating is not an option: communication must positively occur.