What could be done as early as historically possible to negate the effects of plague and disease? I'm wondering this after seeing the death tolls from the Plague of Justinian and the Black Death, and finally realizing just how much it screwed the potential of the Mediterranean and shifted power northwards. I'm not asking for miracles to occur for both of these consecutively, but I am wondering what could be done in general to limit disease as early as possible, in a manner realistic with what was available at that time? What could have been done that wouldn't rock the boat drastically in terms of POD?