There are three different types of hitpoints: soft (SHP), hard (HHP), and maximum (MHP).
SHP reflects current hitpoints. As long as a character's hunger is above Very Hungry, this value will slowly regenerate, up to the character's current HHP value. Being reduced to zero SHP results in being knocked unconscious for roughly a minute.
HHP is current maximum hitpoints. This number does not regenerate naturally. Leaving your character to starve will make you lose HHP over time. HHP only recover by healing with leeches or gauze. HHP has a maximum of the character's MHP value. A character takes slight HHP damage from most attacks, unless unconscious, in which case all damage goes directly towards HHP. If a character's HHP drops to zero, that character is permanently dead.
MHP is the maximum achievable hard hitpoints. This value is 100 on a new character and can be raised or lowered in only two ways. Raising a character's constitution attribute will increase MHP, while lowering it will decrease MHP. This includes the CON bonus/penalty from changing the Barbarism/Civilization belief slider. Shifting the Life/Death belief slider also directly changes a character's MHP by up to 20% in either direction.
One way to heal HHP is by using leeches. Leeches can be obtained by anyone, regardless of skill, making them a popular low-level healing method. However, because they deal damage, their use is not recommended at very low SHP values, as the leeches can knock a character unconscious, which would lower HHP instead of healing it.
Leeches can be obtained by walking in any swamp area. They will attach themselves to empty equipment slots. The less a character is wearing, the more leeches can be gathered and used at once. Once a leech is equipped, it will do SHP damage about once per minute. The damage it deals is equal to its quality divided by 10, rounded down to the nearest whole number. Approximately every 10 SHP damage taken will restore a single point of HHP. After damaging a character three times, a leech will become bloated. If removed and stored, bloated leeches will eventually become normal leeches, allowing reuse simply by placing them in empty equipment slots.
The quality of a leech moves one point towards its host's constitution attribute each time damage is dealt. This means a leech can gain up to three quality points per usage. As such, reusing leeches can result in better healing during future uses.
The second way to heal HHP is by using Gauze. Gauze provides healing at no cost to SHP, though the skill requirements to obtain the necessary wool is considerably higher than that of the leech method. However, because gauze can be applied by anyone with the First Aid skill, it is possible for a character to give another character assistance.
To obtain wool, a character must either own a tamed sheep or find a four-leaf clover and a mouflon. Using a clover on a mouflon allows shearing, while a tame sheep can be sheared without clover usage. Once wool is obtained, it can be made into gauze at a loom by any character with the First Aid skill. Once created, gauze can be applied by left-clicking the item to pick it up, then right-clicking the desired target and choosing to apply the bandage. Gauze can be only applied to a character that has no headgear equipped. Gauze will heal a character's HHP by 30% of their MHP over time.