I am showing my earlier comment as a new post:
I think that MDs command respect because of what they had to go through to get into medical school.

First, if you look at the statistics for Keck School of Medicine Class of 2006, you can see what is required to be considered for admission:
Total Applications (2001) 4,407
Interviewed 451
Expected to Enroll 160
Mean GPA 3.62
Mean Total MCAT Score 32
Source: http://www.usc.edu/schools/medicine/school/about/facts_figures/index.html

Second, teachers or public health professionals are not usually given this respect because their training and education are not considered as rigorous as the training of medical doctors. To become a medical doctor it includes: getting an undergraduate/graduate degree, graduating medical school, passing all 3 steps of the United States Medical Licensing Examination, going through residency, and so forth.

Third and lastly, this high status and respect for doctors is deserved because of the education that they have to endure to become a doctor. However, all medical doctors are human and they are prone to mistakes just like everybody else. Hence, they should NOT be considered all-knowing and should be questioned and challenged by their patients.