Let me first say that you can’t make someone believe anything they don’t want to believe so if you want to try to get them to see your side and believe differently than they do I would suggest doing extensive research and not just research on websites that are not reliable go to actual websites from doctors and hospitals etc. and even ask people with the same disease if you can.
PS don’t down the person for their beliefs or tell them that they’re believe are wrong because you don’t know the reason for their belief Just help them to see the truth but don’t push them from it