Eventually, you suffer periodic damage which will cause death. What's Different in Fallout 4's Survival Mode: Things are generally bleaker and more unforgiving, and every enemy's lethality is significantly higher.
Survival difficulty replaces the normal Fallout experience with something significantly more nightmarish. Unless you've been living under a rock, blind-folded, with your fingers in your ears, you've probably heard about Survival difficulty in Fallout 4.
