Survivors/Stats/Fortitude
Fortitude is one of the six stats a character has in Sheltered 2. It is symbolized by a green heart 
icon.
About
Fortitude is one of the three combat-related skills in the game, alongside Strength and can be considered one of the most important stats in the game, as being able to tank bigger hits is
Uses
Fortitude affects a survivor's maximum health. This effect also applies to wasteland strangers, faction members, and aggressive animals.
Fortitude also affects how long radiation sickness lasts on a survivor, as well as how long a illness lasts. These two effects only apply to playable survivors, though.
Gaining XP
There are two ways to gain fortitude experience. The first is to have survivors exercise using punching bags and weights. Training using punching bags and weights will also grant 
Strength. There are four tiers of Fortitude + Strength training objects: the Light Punching Bag, the Light Weights, the Heavy Punching Bag, and the Heavy Weights.
The second way is to have is to have survivors exercise, this time using treadmills and exercise bikes. Training using treadmills and bikes will also grant 
Dexterity. There are four tiers of Fortitude + Dexterity training objects: the Treadmill, the Poor Bike, the Steep Tradmill, and the Exercise Bike.
Level Caps
As with the other six stats, the level cap for Fortitude depends on the character's starting Fortitude level. The higher the character's starting Fortitude, the higher the level cap will be.
Leader Traits
As with all other stats, Fortitude has leader traits that adjust its maximums, or give extra Fortitude.
If you give your faction leader the "Unbreakable" leader trait, then they will start with a guaranteed max Fortitude of 20. This trait costs 3 trait points to apply. Additionally, if you give your leader the "Sturdy" trait, they will gain +3 levels in Fortitude.
Conversely, if you give your leader the "Flimsy" negative trait, then they will start with a guaranteed max Fortitude of 10. Adding this trait will add 1 trait point.