I think I've got a pretty good theory regarding the story behind the zombies:
I believe Russman is eating the zombies.
What first made me think this was the quote:
"I think I'm getting cataracts, this is a restaurant isn't it?"
A play on words where he's referring to "I'm getting" in the sense that he's ordered food, as opposed to contracting the disease. I believe he's talking about zombies because cataracts causes a clouding of the lens inside the eye, which produces a similar look to the zombies eyes.
There's also the quote:
"Them steak-stumblers are gonna cook from the inside out"
And I don't know about you, but unless I was eating them, or considering it, I don't think I would refer to the zombies as 'steak-stumblers'. In fact steak is probably the last thing I would associate with them.
My next clue was Marltons quotes from the bunker in Nuketown:
"Just because one is different is not a valid excuse to simply banish him."
"They ask me to leave? I was the one reporting this scuffing behavior... makes no sense."
Perhaps the behavior is eating zombies?
Then there's the "STOP RUS" at the bus depot, could that be in regard to eating zombies?
There's also the logic behind it, they're in a post-apocalyptic world where there is a food shortage. I've heard others mention that Russman's character appears to be homeless, a group of individuals who sometimes eat questionable things anyways.
