As far as I'm aware, your initial Health stat is based upon your starting Endurance stat and your maximum health increases each time you level up also based on your endurance. Some of the higher powered guns do have the ability to one shot kill you when you're at a low level, but you usually don't run into people carrying them outside of guards until later in the game unless you're super unlucky, instigate fights or go to a later game area early. Weapons have a damage range and won't always hit your for their maximum amount possible. You can also aim for certain body parts, some of which can damage health more than others or have secondary effects I think, though not sure how often the AI does that. Once you leave the ATOM base starting area there is a scripted fight you're expected to lose where you get knocked out and robbed of your starting gear but you don't die (and with a specific character build, tactics and some luck it is actually possible to win it). After that most battles will go on until you or your opponents die, and if you die it is game over and you'll have to reload a save, but generally enemies you face should be within your ability to beat most of the time if they start the fight. Along with health increases as you level up you're also able to find, trade and craft armor to offset incoming damage so you can take even more hits.