Treatment and diagnosis are both critical parts of healing the mind and body. They are relatively broad terms. Simply put, diagnosis is the identification of a problem by considering its symptoms. Treatment is the manner in which someone goes about handling the problem. Both are important parts of dealing with ailments, stress, and wounds.
When a character sets to taking care of another, they first have to figure out what is wrong. It may appear obvious in some cases; a massive gash down someone's leg for example, is a very obvious wound. But diagnoses can come out with details which may or may not affect how to treat the wound. If the character realizes that their wounded friend has cut their femoral artery, for example, they might try to cut of circulation further up the leg rather than applying pressure to the site of the wound itself.
Players and storytellers do not have to know exactly what to do for every wound, disease, or stress that their characters come across in game; no medical degrees are required to play this game! A diagnosis is done by rolling a (Mind + Nature/Mysticism) check at a difficulty determined by the storyteller. Upon success the character will receive information about the condition of their patient from the storyteller, and know what the next steps need to be in treating the condition.
Failing a diagnosis roll does not mean that a character cannot treat another; rather they just do it blindly.
Treating conditions is done with (Mind/Spirit + Nature/Mysticism) for the most part. The Survival skill can be used in some cases for wounds and ailments. Characters can use social skills to help others through mental distresses. While most conditions can be stabilized by almost anyone, many require specializations to actually go about treating.
Most ailments require specialties within a character's Discipline path to effectively treat. Furthermore, most also require equipment and materials in order to use the specialized knowledge to actually help a patient. Major and Grave wounds also require specialties to treat effectively.
The first part of treating a character is to stabilize them; dressing wounds, mitigating symptoms, setting bones, etc. Anyone can attempt to stabilize a character suffering from the effects of a condition with a standard treatment roll against a difficulty set by the storyteller. Success means that the character is stable; they will not bleed out, get an infected wound, or spiral into a lasting depression. In the case of someone suffering from an ailment, this might mean simply keeping them comfortable and nourished.
Stabilizing a character is only a narrative victory; they still suffer the effects of the condition, but it will not worsen into secondary conditions, or death.
The Treatment Process:
Each condition is a different beast, and each patient is different too. Therefore the process of treating each condition is vastly different. Depending on the condition, there is an amount of time it takes for a character to recover. Treating a character generally helps the patient recover faster, and prevents them from sliding out of a stable state.
Storytellers will ask for one or multiple treatment checks as the character sets about their work. Success reduces the recovery time of the condition to its minimum, and keeps the patient's condition from worsening. Failure on a treatment check will lengthen recovery time, and patients will have to make a (Body/Spirit + Endurance/Poise) check to remain stable. If the patient becomes unstable again they may acquire a secondary condition, or start losing Health. Failure to re-stabilize them can likely snowball into death.