Medieval Beliefs About What Caused Illness
Medieval people had three main explanations for why someone got sick, and they often combined all three rather than picking just one.
The Theory of Four Humours was the most scientific approach available. Based on ancient Greek ideas from Hippocrates and Galen, it suggested your body contained four fluids (blood, phlegm, yellow bile, black bile) that needed perfect balance. If they got out of whack, you'd fall ill. Physicians would literally taste your urine to diagnose problems - imagine that job interview!
Miasma theory blamed "bad air" from swamps, rotting corpses, or filthy conditions. This wasn't completely wrong - many diseases do spread in unsanitary areas, though medieval people didn't understand the real reasons why.
Supernatural explanations dominated most people's thinking. The Church taught that illness was either punishment for sin or God testing your faith. Since the Church controlled education and most people couldn't read, these religious explanations felt more believable than complex medical theories.
Think about it: Medieval people were actually quite logical - they used the best information available to them, even if we now know it was incorrect.