For starters, this wouldn't make for a great campaign unless your players are happy to swap out characters willy nilly because if you did more than four adventures (or levels), it's unlikely that any of the beginning characters will survive to the end. Heck, even with four adventures this is unlikely. Ensure that they can bring in new characters at the safe house that marks the end of each adventure.
The easiest way to think of this game design is an urban dungeon crawl with lots of zombies where certain traps (car alarms and other noisy occurrences) will cause a horde of zombies to rush forth. Create a street map with a few building maps to keep on hand and have a think about which doorways to board up, which cars to alarm, and where to place ammunition, health packs and safe houses.
Car alarms will be a tricky one. Some games have the option to hit the person or object behind the target if you miss the target by a certain amount. In other games you'd only hit the car if the zombie were accidentally using it as cover and were partially concealed behind it and you either missed them or tried to shoot through the car. If you're using a miniature map (which may prove problematic due to the size of streets as opposed to the size of your average dungeon room for which they are intended) you can at least reveal there's a car behind them that *might* be alarmed. Otherwise you'll have to describe it as being there and give the players a choice.
If you want a desperate scrabble for ammunition, sprinkle it around and develop ammunition item cards where the players can erase bullets as they go - either by adding them to their revolvers or once the clip itself is spent. This isn't too much minutiae *if* you create an easy system with erasers. Otherwise, it probably will be and might also damage the character sheet if you're using that sheet to rub things off.
I'd recommend introducing health packs although very few roleplaying games use them (unless you count healing potions in D&D and Pathfinder). The player characters will be taking a lot of damage and it's important to give them something that will allow for quick and easy healing. Simply dropping Cure Light Wounds potions that look like first aid kits into your game would work a treat. Make it take a round to use on yourself or someone who has been downed. If you want to be true to the game, allow player characters who have been dropped to the floor a few rounds to be saved by a health pack before dying.
For those who don't know, a Cure Light Wounds potion heals 1D8 hit points. You could have something similar to heal World of Darkness health levels, Call of Cthulhu hit points, or any other game's health meter.
Oh, and it's probably also a good idea to let your player characters get more hit points than usual as they'll be in a lot of combats. If you tried this game with regular World of Darkness you'll get frustrated by players that become obsessed with stealth and hiding. When you try to flush them out with hordes they would then get frustrated as well since they have no hope of surviving. You want a nice balance where they should be able to take on several zombies and a special zombie in a single encounter without dying for good. If this means giving them more health levels, perhaps double or triple, then that's fine.
|Some people have dogs. I have Special Infected.|
These are also not your usual shamblers. They're faster than most and some of them are *really* fast. They can run, jump, climb, and break in through windows. The only place they won't break into (or surround) are specially designed safe houses that were meant to protect people from the zombie plague. Some of these zombies have special powers - they puke on you which attracts zombies, they have long tongues that can entangle you and drag you towards them, or they can charge like bulls and knock you down.
Oh, and one other thing. The player characters need to be immune to this disease. If not, they will likely be infected within the first encounter. Yeah, this game has that much combat in it and any roleplaying game that tried to use the same framework needs to have the same.
Explorers will enjoy the opportunities to take a sneak peek into a number of real world locations so try to be a bit imaginative and perhaps do a bit of research into the sort of settings you use - such as IKEA. Try to get your hands on a map, at the very least. These players might not be so keen on the restrictions placed on Exploration, though. After all, the risk of another zombie horde attacking discourages poking about in drawers or spare rooms.
Investigators won't have much to do here. If they're a big fan of zombies they might be okay for the short-term but will likely get frustrated since all of the running and hiding in safe houses doesn't really give them the opportunity to hunt for clues and exercise their mind getting to the bottom of something. If they tilt towards being one of the Tacticians or Action Heroes on a secondary level than they might be all right. Otherwise I'd recommend more of a Dead Island game.
Communicators will need to get their kicks from safe houses so try to introduce the odd NPC - perhaps over radio - so that they don't feel stifled by a complete lack of social interaction. You could even introduce the politics of safe houses if you were willing to throw in some elements of Dead Island as the various survivors compete for resources - namely your assistance. Certainly give them a breather in the safe houses to talk amongst themselves which will be popular amongst all of the roleplayers and dramatists anyway.
If you want to check out the trailer, you can find it here. There's also some TV Tropes that you can take a look at over here.