These skills deal with different areas of treatment. It is important to note that while a character may have all of these skills, they can only use one at a time.
This system has been designed to make a single ‘Healer’ almost impossible. The minimum time required to return a hit is the same time it takes to drop a hit. You can pause the Death Count by using First Aid, but you can’t use another skill at the same time. As such, the realistic minimum required to perform healing is a ‘doctor’, with the Medic/Surgery skill, and a ‘nurse’ using First Aid to assist them. This way the nurse can be holding the Death Count while the doctor heals.
You will notice that this makes performing healing ‘in the field’ (during combat or away from the settlement) very difficult. This is also intentional, as the Wilds are dangerous and death can come easily. Easy on-the-spot healing is not very realistic and we feel that it reduces the atmosphere of the game.
Hopefully the most likely outcome of this is that players set up clinics; areas injured people can be brought to for healing, presumably for a fee. Medical issues can then become a business and, for example, selling single use medical goods (e.g. anaesthetic that removes a red bead during surgery) then becomes a way of keeping the economy flowing.