There is a phrase I just love. "Absense of evidence is not evidence of absense". You're right, most people think that if there is no evidence to prove something then it must not be true.
Nothing is further from the truth. Just because there is no evidence to prove that someone did something does not mean they are innocent of the act. It just means that no evidence was found to prove this person's guilt.
As for the "doctor knows best", I think that is dying a natural death. At one time, doctors were like gods, their word was law; but, I think that people are taking a little more responsibility for their own health today. Plus, with the internet, etc. we have much more access to medical information.