Probably the toughest challenge in having my fantasy world depicted as realistically as possible is to define how healing works. So far I managed to establish several key rules, but I can't help but feel there are some gaps in the logic that could be exploited.
First and foremost, I don't want my healing system to sound like pseudo-science or an ambiguous magic techno-babble, I want it to sound as sensible as possible. Its first limitation would be that it requires an exorbitant amount of energy to take effect and that energy has to be provided by the creature that's being healed rather than the healer. In my world, the healer would simply be the conduit and the catalyst, his energy input would be the guiding burst that initiates the healing process and maintains its performance.
Now, to prevent pretty much everybody from being unkillable as long there are healers around to provide healing and imposing how genuine a thread death itself is, I altered my healing to have steep price. Rather than requiring mental focus or spellcasting capacity from the creature that's being healed, healing actually draws directly from that creature's life source. In short, whenever a creature is healed, it has to pay the price by losing a part of its lifespan equal to the amount of healing done to it.
Also, to prevent my world falling into the MMORPG sinkhole of having the healing too powerful and able keep up with damage being dealt in combat, I increased the time it takes for it to be applied. I still have to determine the exact pricing on the healing itself, but so far I managed to establish the following guidelines:
Healing minor wounds, such as minor cuts, bruises and dislocations that would heal on their own within a week or two would shorten the lifespan of the creature being healed by merely a few days. This kind of healing could be done under a minute in a single session and wouldn't be strenuous to the creature being healed nor the healer.
Healing serious fractures and wounds that could lead to serious conditions if not tended to would shorten the lifespan of the creature being healed by weeks up to a full month or two at most, depending on how severe the case is. This kind of healing would require several minutes to get done and it could provide an additional session of healing in order for the creature to be fully healed. This would exert minor exhaustions both on the creature being healed and the healer.
Fatal injuries and fractures that could prove lethal if they aren't quickly resolved would shorten the lifespan of the creature being healed by months, even up to a year. The first healing session would have to be performed just to prevent the loss of life and it would greatly tax both the creature being healed and the healer, not to mention it would require expertise from the healer to be properly done the first time. In order to fully heal the creature from this kind of injury, the healer would have to perform several sessions with at least one day of gap in-between them to allow the creature being healed to handle the exhaustion of the process.
Recovering severed but nonessential body-parts (arms, legs) or recovering long-term injuries that left the creature in a debilitated state would require a tremendous price both from the creature being healed and the healer. Not only would this process cut well over a decade from the lifespan of the creature being healed, but it would also crank its regeneration rates into obscene levels, which would exert tremendous exhaustion on the creature, not to mention it would require the very expert healers to be performed in the first place. In order for the process to begin in the first place, the creature being healed would have to be dropped into an induced coma, only to awaken several days or even weeks after multiple healing sessions have been applied by the healer.
According to these rules, it stands to reason that the more a particular creature is being healed, the shorter it lifespan becomes. This means that higher forms of healing represent a serious investment and the creature being healed has to weigh its options carefully on the long run between having a more comfortable or a longer life while also putting the healers in quite a prestigious state.
As far as the healers go, since they are the conduits and the catalysts of the healing process itself, their lifespan isn't affected by the healing process, unless they are the ones being healed, of course. The only strain on the healers, when doing the healing of others, is the strain on their own energy and their expertise.
The second hurdle with realistic healing would be to differentiate what can be and cannot be healed. So far I managed to define the following:
Standard healing is applied in order to strictly enhance the regeneration rate of the creature being healed. This refers to negating basic physical injuries which could be sustained in combat or under normal working conditions. Any sort of healer can apply this sort of healing, the only difference is that more skilled healers can apply more of it more effectively.
Afflicting equates to applying counterproductive effects on the creature, diminishing its performance rather than enhancing it. Healers are actually the ones who have the least of these capabilities at their disposal, since the actual combat specialists are the ones who use these the most. This sort of "healing" has no efficient applications outside combat... except for the purpose of torture maybe... or sadism... or whatever other diabolical stuff one could come up with to pass the time...
Boosting is applied mostly in combat and it refers to temporary enhancing a particular set of capabilities of a creature other than the healer. While most spellcasting combatants have the capabilities to enhance their own performance through BUFFING, the healers are mainly the ones with the ability to apply those enhancements on others. There are numerous sorts of enhancements and a healer can only apply a single type of enhancement on his target through a system that pretty much functions like the Concentration system in Dungeons & Dragon's 5th edition.
Cleansing is applied to combat a particular disease that's affecting the creature. The main downside of this healing is different types of diseases demand different sort of Cleansing. This sort of healing cannot be done blindly, the healer has to be fully aware with what kind of disease the creature is being affected in order to combat it. If the healer applies a form of Cleansing that doesn't comply with the disease he wants to treat, it could lead to complications and even to death. Therefore this form of healing is mostly done by the experts and lower-tier healers tend to avoid it altogether.
Immunisation is applied to temporarily enhance the creature's immunity. Most of the time this boils down to forcefully manipulating the creature's body temperature to deal with minor diseases that could be naturally dealt by the body itself over a prolonged period of time. Because of its simplicity, any sort of healer can apply this sort of healing, the only difference being is, again, that more skilled healers can make it more efficiently and comfortably.
Recovery is applied to restore the spellcasting capacity of one creature by granting it the spellcasting capacity of another. In short, the healer would relinquish his own energy to grant it to another creature (basically it's a direct energy transfer). Other than applying this sort of healing to force the comatose back to consciousness or recharge other combatants in the middle of combat, this sort of healing has no feasible use. Needless to say, the healers are the only ones capable of doing this and even the most basic healer can do this.
Regeneration is applied to recover severed non-essential body-parts and it stands as a category of its own already mentioned above. Its main downside is its strain on both the creature being healed and the healer, along with the tremendous price the creature being healed has to pay for it. Finally, expert healers who specialize in this sort of healing are far too few around, therefore such expertise are in quite a high demand and quite on a high price. This kind of healing also applies to "fixing" irregularities, for example if the creature's bones have grown back the wrong way they first have to be broken apart and then regenerated. Just like with Cleansing, Regeneration cannot be performed on a generalized blanket system and requires intricate knowledge of the creature's condition.
Revival is applied to return a fallen creature back to life. It's the most prestigious capability of the healer and only the most profound healers are capable of doing this. Due to the ways the world functions, this process is too complicated to briefly explain and it requires a subject of its own, so I'll keep it brief. The longer the creature stays dead the more of an energy investment is required from the healer to revive it, every passing minute of death means the price of revival increases exponentially. While I haven't clearly defined how far a creature can be dead in order for it to be still successfully revived, I'd say that about the time decomposition kicks in there is practically zero chance of reviving the creature, unless the healer is of an ultimate tier and is capable of seriously bending the rules of reality. PRESERVATION actually falls into this category as well since it directly affects its effectiveness. Preservation basically slows the decay of the body and allows for more time before Revival can be applies, it functions somewhat like Gentle Repose in Dungeons & Dragons 5th edition but it does not negate the decay completely.
Needless to say, Revival puts an outrageous strain both on the healer and the creature being healed, all the while demanding a ludicrous price from the creature being healed in the process. Depending on how far gone was the creature before it came back to life, its lifespan would be reduced by at least several decades, sometimes even more than half of its entire lifespan.
Most notably, in order for the Revival to be possible, the healer has to have most of the creature's original body to work with. The more of the creature's body is missing, the more difficult the Revival becomes, especially if the vital parts are missing. It goes without saying that Revival is impossible if the creature's head is missing...
Instead of the creature's lifespan simply being cut short by the amount expended in the healing process, the healed creature would actually experience accelerated aging. For example, if a person that was killed in their early 20s barely managed to be brought back to life, it would appear as an old man in its early 30s and would not be able to reach 40 years of age.
Feel free to help out with the idea, I'm open to improving it further :)
PS EDIT: The reason why I'm posting this as a question is that I need feedback to make sure the system I'm developing seems as realistic as possible and it couldn't be exploited because of a technicality I might have overlooked.