Medical-nin are shinobi who specialize in medical treatment through the use of the branch of ninjutsu known as medical ninjutsu. During the Second Shinobi World War, Konoha was suffering heavy casualties in the battlefield due to a lack of proper medical treatment available to fallen combatants. Because of this, Tsunade, who would eventually go on to become the world's greatest medical-nin, spurred by the death of her own brother suggested that certain shinobi be trained in the medical arts and then be incorporated into regular platoons with the goal of increasing mission success and survival rates. In time, this was fulfilled.
Medical-nin are required to be highly intelligent and must have excellent control of their chakra. They are not allowed in the front lines and are trained to focus on evasion so that they can live long enough to ensure the survival of their teammates. With this in mind, Tsunade created four rules all medical-nin must abide by:
1. No medic ninja shall ever stop medical treatment until the lives of their party members have come to an end.
2. No medic ninja shall ever stand on the front lines.
3. No medic ninja shall ever die until they are the last of their platoon.
Being the only person to have mastery over said technique, Tsunade is the only medical-nin permitted to participate in direct combat. She trained many of the first medical-nin and made great contributions to the field, which eventually led to Konoha's victory in the war.