Its many things.
Its also not random. The major sources of unhealth are:
- being too hot or cold
- The first level of too hot or cold will only hurt your current health. This is 'chilly' or 'warm'. The next ones will hurt your longterm health.
- I dont change this, its already present in the base game.
- Poison will drain your health by quite a bit. Poison resistance will help a lot, but poison will make you less fit, period. This applies to food poisoning as well.
- Crappy (non)foods
- Taking lawn fertilizer isnt a healthy thing to do, nor is eating junk or impure water.
Its a scaling system that is being tweaked. But in general negative health isnt a untenable position. In newer versions its looked at. Specifically the health loss by temp. its nerfed down quite a bit.
Stay comfortable, be aware of what is and isnt poisonous, and dont eat poorly.
And dont trouble too much about health; its less of a hindrance than you think. Its a passive and relatively unimportant stat. Unless it is at -200.
In my playthrough on youtube my character's health is -50 and hmod is -150, and hes doing good but any standards.