The sub-story behind Zombie Stress is not crucial and doesn't really need to be followed at all. It's just there for atmosphere anda bit of fan fiction.
The Sub-Story
After Gordon Freemans adventures through Ravenholm, the Xen Zombies are now looking for the meat which they have craved for so long. The once tough gates of Ravenholm have been broken and they have begun a mass sweep against mankind and the combine.
You are Gordon Freeman, once again compeled into a world that needs your help. Prior to your second involvement with the G-Man, [so before Half Life 2: Episode One], there is an error and you are compelled into an outlet of the Zombies. A once combine controlled area. You have to battle to get in deeper into the Combine lines and then out again in order to get as far away from the Zombie hords. But the Xen Zombies are not going to let you out lightly.