Suppose that the Miasma theory of disease took off during the 800s and by 1100s, 75% of doctors thought it was true for most diseases and the other 25% can have... any really. The 4 humors or something. However, the idea of letting pus drain is still deemed a good thing (just like in OTL when Miasma theory came, they still thought letting pus fester under your skin was a bad thing because... it is).
So what does this change? There will be a lot less blood letting and a lot more incense.