They absolutely believe they are telling the truth. If you try showing them their error, they get lots of negative feelings (sadness, shame, anger) and need to remedy the problem. The problem is you. Why did you tell them these things that made them feel so bad?   Clearly you are wrong, and do not know anything about medicine (despite being a doctor).   Clearly you are hateful, and do not want them to feel good about themselves (despite being married to them).   Clearly you are biased, and simply trying to spread lies so you feel better about yourself.   But the problem is their incorrect perception of reality, which increases their risk of death.