Yeah, actually, it would probably be pretty easy to explain germ theory to a Medieval person as tiny evil spirits that live on everything, but they can be purified by soap and water, or by alcohol, because that is why God has granted us those things. And because they can float in the air, if you cough or sneeze after they have infested you, that can cause them to infest others. And when you are sick, the angels God has deputized to defend the bodies of His beloved children are at war with the evil spirits, and, sadly, sometimes they lose, but the best way to help your angels win their battle is to rest, drink plenty (this would probably be small beer in this time period, not water, because the water was also infested), stay clean, and for the sake of God do not allow anyone to let your blood, for the angels need that blood in their war against the evil spirits. Bloodletting is good for some types of illnesses but not the kinds caused by the tiny evil spirits.