Simply amazing
I would have never thought something so simple yet fun could be made in flash, reminded me a lot of Dead Rising for obvious reasons. There's not many suggestions I can make to improve it besides lengthening it from 20 days obviously. What I really mean is, this is great but if you considered making similar games in the future consider more weapons/realistic upgrades for weapons (earned by kills or credits collected from different types of kills, standard kills won't be worth as much as headshots for example. But as far as upgrades are concerned consider makeshift bayonets, double reload speed with double taped clips for the assault rifles/machine guns, shotgun shell belt, etc.), maybe even the ability to arm survivors with discovered weapons. (Every survivor with a Barret = Nothing getting close.)
But there's not much you can do to improve what's nearly perfect, excellent job, I believe I would even be willing to somehow personally sponsor a sequel, if that would be what it takes. 10/10