In the early centuries of the modern university in the west, medicine was one of just three advanced (PhD-level) faculties: law, theology, and medicine. In addition to the university-trained physician tradition, there were numerous levels of medical practitioner from royal surgeon to urban apothecary and battlefield barber-surgeon to village herbalist. In addition to the wealth of formal medical texts inherited from Greek antiquity and the medieval Islamic world, increasing literacy in the late Middle Ages brought practical handbooks of medical knowledge into the hands of medical practitioners with less formal education.

The writings of Galen, Aristotle, and Hippocrates (that Hippocrates) underlay the theory and some of the major treatment routes in the Latin west. Of course, their shared "four humours" basis of human biology was completely wrong as we understand today. But ancient and medieval people did not. And so medieval medicine--treating diseases and disorders--was systematic and logical, but based on an incorrect framework.

Hence, for example, the much-maligned leeches and bloodletting treatment. If you believe that disease manifests itself through an imbalance in humours circulating within the body--too much bile, say--then bleeding out some of the bile will help restore the balance! Other means of purging, like enemas or blowing one's nose, could be construed similarly.

Don't get me wrong, there's a lot of stuff in medieval medical writings that is a MAJOR head-scratcher today. But that doesn't mean that doctors were universally stupid or ill-trained. In fact, there is a long, strong tradition of medieval literature satirizing/complaining about quacks. Which is to say: medieval people definitely understood what a good and an effective medical practitioner was and could do, because they knew enough to denounce the bad ones.