The standard medieval means of treating injuries only did so at a slow rate--that is, if they did so at all. The perfection of surgery techniques allows for the more rapid treatment of injuries and the ability to heal many injuries previously untreatable. Lacking hospitals, surgeons are usually available instead at any (mostly defensive) buildings that can be garrisoned.
However, such improvements didn't reach the Bretons or Guelders. It would thus not be a good idea to go in for treatment in either of those two countries.
Researched at: University
Cost: 350 florins
Effect: Units garrisoned in buildings heal 4x faster