Oh good, I'm glad I didn't have to be the first one to do this. :P
Like Bucky said, the writing is very good, and I still like the whole concept of the story, but the fact that these people are still alive at all after five years in those conditions would require such a suspension of disbelief it would really bug me.
The major one of course is water. Everyone is going to need a lot of water, every day. I'm not sure where 'bread and scraps of fish' would come from. Even the most processed, preservative packed bread is going to last a month, at most. Fish I guess could come in a can...really canned food across the board is the only option that would make any sense without any power or other means of preservation. And the other big issue would of course be hygiene and waste disposal.
Drinking nothing but the occasional bit of contaminated water that trickles in under a crack in the door, and having no sanitation whatsoever means these people are [i]all[/i] going to be sick and dying in far less than five years, and not neessarily even from whatever zombie plague is going on outside. (Though with them tramping around on the ground your water flows in from, I guess I wouldn't rule it out...)
And that's not even going into the mental toll five years of that would take, even if survival in those conditions was somehow a possibility. Hard to believe at least one person wouldn't have completely cracked, committed suicide, etc.
Is there any particular reason life for them has to be so dire? Seems like they'd be so miserable on a day to day basis the threat of the disease getting in wouldn't have as much impact with so little left to lose. Most bomb shelters have bottled water, canned food, batteries for lights and a radio, a toilet, a generator, maybe even some books or cards or board games. (Most people have some of these things in their houses as well that it would make sense to have brought along...) Not exactly a happy existence for five people crammed in for five years, but it's something.
If the entire point of the darkness is so you don't know who coughed, it would still be dark at night when it happened, or maybe the lights all running out is a more recent thing, or even a deliberate thing.
Weirdly enough, the more I think about it, the more interested I am in the psychology of these five people slowly cracking in a confined space as the last batteries in the last flashlight slowly die, and then turning on the guy who gets bronchitis and murdering him. :D Whether he even had the disease or not would actually be irrelevant at that point.