You were infected, bitten by one of the zombies while on a mission. Every day it got worse, but Doomguy just couldn't let you die. He hated it, the fact he'd lose another after losing so many before.

Your conscience was barely there as he entered the room. Everything hurt, it was hard to see, hard to understand what was going on. All you felt were his light hands, carefully bringing your forehead to his. Trying his best to calm you down, to keep this infection from getting worse.