((awesome, Though honestly I think the huge faction Idea will put off many people. Though, if you insist on bringing it in, it would likely be the military, because they are already somewhat well equipped to survive an apocalypse (look at NORAD, Cheyenne mountain, etc). AND they already had the builders, doctors(Though they specialize in stabilizing stuff like bullet wounds, explosions, and burns. So if a virus hit them, they wouldn't have the supplies or the know-how to fix it without asking for help. They likely won't have many farmers either, which could be used to balance this out (they would likely have years and years and years worth of food stored, but we can ignore that for the sake of balance).
We could also have a group of normal civilians, a good chunk of which were farmers, so that way, they would have an excess of food, but likely little in the way of firearms, because they're civilians. They would also probably have a few foreman in their midst, so they would well fortified. they would have a moderate amount of doctors and supplies as well, seeing as their territory encompasses two medical clinics (But no real hospitals).
Then, lets say in a big city somewhere, a group of police officers was ordered to guard a hospital until helicopters came to pull them out.
They didn't.
So, they stayed. fortified up a hospital as best they could with their meager knowledge of carpentry, scouted out the city, and, finding only one or two survivors, claimed it as their own.
The populace of the city would likely have relatively poor relations, as far as diplomacy goes, with the military. (they did abandon them after all.