Actually, Diamondz, the same question is just five below this one. Having got there, I see that Shaney recommended exactly the same website for an answer as I did above. Great minds think alike!
Early Medical professionals practiced medicine based on both fact and fiction
In the early days it was thought that the common duck was a magic bird that could prevent infection.
Doctors to protect themselves from disease would insert a malard into their anus prior to visiting a patient and when they bent over to exam the patient the pressure on the mallard exerted throught the anus would cause the bird to quack hence the term quack is used to describe a doctor.