Harm and Healing

You live in a dangerous world, pocked with tiny niches of relative safety and community. Traveling between these isolated settlements is risky at the best of times; there are no roads, only worn game trails barely wide enough for small carts. Hungry, territorial, or sickened wildlife may contest your passage, and the flora is hardly gentler with its traps and thorns. Other people are rare outside their own home ranges, so help is unlikely should you find yourself endangered or injured or ill.
Perhaps before the Rupture, there were ways of rapidly and fully healing from wounds and debilitating conditions. However, here and now, even a minor injury can turn fatal if not properly and promptly treated.

Character Health

General Unwellness

It is possible to get hurt or sick without reducing your Health. Minor and temporary afflictions that naturally heal over time, like an overnight fever or a bruised elbow, can be accounted for by modifying relevant rolls. An acute headache may impose disadvantage on your Wit rolls, while a mild cold may reduce your Fortitude rolls by 1.
This is intended to be a flexible system driven by the narrative, unlike the mechanics below.

Stress Points & Harm

You have two mechanics that measure your physical health and wellbeing. When you are dealt harm in combat or in dangerous situations, it will either produce a widespread effect (like falling from a height or getting knocked down a slope) or it will be localized to a specific body part (like a bite to the arm or a kick to the ribs).
Generalized physical damage registers as Stress. You can suffer up to (2 + your Fortitude bonus) points of Stress before risking short-term incapacitation (usually being knocked out). A singular event may inflict 1-3 points of Stress, based on its severity. If you have exceeded your maximum Stress points, any further Stress will deal Harm instead.
Localized damage registers as 1 Harm to that specific body part (limbs, head, torso, tail). Any given body part can suffer up to 3 Harm before being permanently destroyed.
You can suffer up to (4 + your Fortitude bonus) total points of Harm (across all body parts) before being incapacitated and risking death.

General Healing

Recovery can be a slow process and requires sufficient and nutritious food, clean water, and 10+ hours of rest per day/night cycle. (Pushing yourself to your limits, only to collapse in an exhausted heap later, does not count as "rest." Being knocked unconscious does not count as "rest.")
It is possible to break up your periods of rest between a night's sleep (6-8 hours) and one or two daytime breaks (2-4 hours each). In order for these times to count as sufficient rest, you must be comfortably asleep or sedentary. You may do very light activity (like keeping watch, reading, talking, or fibercrafting) during these rest periods.
After a restful cycle, you lose 1 point of Stress, you lose 1 point of Harm from your total, and you may change the status of any 1 body part from Hurt to Healthy. The narrative may reflect continued pain or difficulty even after a body part is technically Healthy again.

Getting Hurt

Each major body part has a status: Healthy, Hurt, Injured, Disabled, Destroyed. One Harm to a Healthy body part changes its status to Hurt, but does not usually carry major risk of incapacitation.
While Hurt, you can still partially use that body part during dangerous situations, and it will heal normally with time and regular care. While Hurt, certain actions that rely on that body part may be made at disadvantage, such as sprinting on a twisted ankle or climbing after dislocating your shoulder.
If further damage is done to a Hurt body part, consequencies are much more severe.

Significant Injuries

Receiving more than 1 Harm dealt to any given body part is enough to inflict a significant injury: a cracked bone, a concussion, or a sprained joint. A significant injury will affect your ability to do some daily things and almost always reduces your effectiveness in combat or dangerous situations.
The points of Harm taken during a significant injury do not reduce with simple food and sleep; medical aid must be given and strenuous activity must be avoided. Depending on the severity of the injury, it may recover within a week; however, if you spend your days running for your life in the wilderness, no progress towards healing will be made and you may worsen the injury, risking disability or inflicting points of Stress on yourself.

Major Injuries

Receiving more than 2 harm dealt to any given body part is enough to risk permanent disfigurement or long-term disability. You may suffer partial or full paralysis of a limb, or you may require amputation; you may need skilled surgery in order to save basic functionality of a limb or organ. Chronic pain, weakness, and/or ease of reinjury may plague that body part for the rest of your life. Mechanically, that body part may never return to a Healthy status again; the best it can be may be Hurt or even Injured.
Recovering from a major injury almost always requires outside intervention by a medical specialist and short- to long-term treatment before you can reduce the points of Harm gained with that injury. At minimum, regular medicine, bedrest, and physical therapy are typically needed for weeks or even months before a major injury can truly begin to mend. Even if a body part is disabled, care and treatment are required in order to reduce the points of Harm gained when it was initially damaged.

Companion Health

Companions are almost always more physically resilient and fast-healing than their partners, having evolved in the high-risk, high-reward environment of the wilderness. As a result, even smaller Companions can survive dangerous situations as readily as their people can.

Companion Stress & Harm

Companions have a maximum Stress total of 1 + their Fortitude + their Level. If they exceed this number, they fall unconscious and risk Harm.
Companions have a maximum Harm total of (1 + their Level) x their Size modifier. If they exceed this number, they are incapacitated and risk death.
The threshold of localized damage that a Companion can suffer to any given body part is also determined by its Size class:

A Tiny Companion goes from Healthy (0 Harm) to Injured (1 Harm) to Disabled (2 Harm). 3+ Harm will Destroy the body part.

A Small Companion has the same damage thresholds as the player character does.

A Medium Companion goes from Healthy to Hurt (1) to Hurt (2) to Injured (3) to Disabled (4). 5+ Harm will Destroy the body part.

A Large Companion goes from Healthy to Healthy (1) to Hurt (2) to Hurt (3) to Injured (4) to Disabled (5). 6+ Harm will Destroy the body part.

A Huge Companion goes from Healthy to Healthy (1) to Hurt (2) to Hurt (3) to Injured (4) to Injured (5) to Disabled (6) to Disabled (7). 8+ Harm will destroy the body part.

Companion Healing

Companions require a similar amount of rest (8+ hours), good food, and clean water in order to recover from Harm and Stress. After a restful day/night cycle, a Companion reduces Stress by 1, reduces Harm by 1, and restores 1 body part from Hurt to Healthy without requiring any special care or medicine.
In order to restore a body part from Injured to Hurt, a Companion must receive basic treatment (usually from its person): a splint, a brace, a bandage, or a salve. With this first-aid, most Companions only take a day or two before their injury is lessened, provided their activity levels are kept light to moderate.
A Companion with a Disabled body part requires skillful medical care, rest, and potentially physical therapy or prosthetics. They are less likely to lose use of limbs outside of an extreme situation and more likely to steadily, slowly heal back to a near-optimal state of health, given time and care.

Game Mechanics

Summary

Maximum Stress = 2 + Fortitude
Maximum Harm Total = 4 + Fortitude
Major body parts: head, torso, each limb, tail
Body part status: Healthy, Hurt, Injured, Disabled
Rest = 10+ hours of sleep or sedentary light activity, food, water
After rest, remove 1 Stress, remove 1 Harm, change 1 body part's status from Hurt to Healthy.

Stress Levels

+1 Stress: A sudden fall, briefly experiencing riptide. Lose reactions for 1 round after the precipitating event. The character may feel slow, stunned, disoriented, or overwhelmed.
+2 Stress: Tumbling down a mountainside, falling down a waterfall. Lose reactions until next rest (minimum 1 hour). The character may feel dizzy, beat-up, panicked, or dazed.
+3 Stress: Nearly drowning, nearly freezing, or being temporarily eaten alive. Lose reactions until next full sleep cycle (minimum 6 hours). Movement reduced by half. When in dangerous situations, must choose between Action and Bonus Action per turn; cannot do both until Stress is reduced.
Exceed Maximum Stress: Results in either uncontrollable meltdown or temporary unconsciousness. Cannot take actions, cannot move, cannot defend.

Severity of Damage

Minor: Tension headache, bruised knuckles. No Harm or Stress points; may add -1 or disadvantage to specific relevant rolls. Will heal with time.
Hurt (1 Harm): Vision-blurring migraine, briefly dislocated shoulder. All actions made with this body part are at disadvantage. Will heal with normal rest and care.
Injured (2 Harm): Broken arm, gaping flesh wound. Actions relying on this body part may not be possible. Must receive skilled medical care and avoid strenuous activity for multiple days to recover from Injured to Hurt.
Disabled (3 Harm): Spinal injury, septic infection. This body part is unusable and most normal activities cannot be performed without assistance. High risk of permanent disability (amputation, severe nerve damage, loss of use). Recovery to any degree is reliant on expert, long-term medical care and oversight and may require dangerous surgery or rare herbs and chemicals to craft medicines. A Disabled body part may be healed enough to be considered permanently Injured or Hurt, but sufficient care will remove the points of Harm gained during the time of injury.
Destroyed (4+ Harm): The body part is permanently destroyed or removed in a physically traumatic way. Don't let this happen to your head or torso.
Exceeds Maximum Harm (total of all body parts): Incapacitated, whether still conscious or not. Further Harm will most likely result in death.