 Prudential norms usually have a hypothetical structure: if you have prudential reason to survive the disease, and if believing that you are going to do so will help you achieve this end, then you have a prima facie obligation to believe that you are going to survive. Likewise, if you want to protect your relationship with your son, and if believing that he is deceiving you and taking drugs will damage your ability to trust him, then you are prima facie obliged to withhold that belief.