Mod? Not sure if there was a mod like this. Virus could always mutate and cause those that were immune to get sick. Instead of eating infected meat to turn into a zombie/infected, a new type of infected AI could spread the infection, however would be cool if you had the option. All that human flesh laying around in PVP maps would have more use as well. I was just including charcoal/antibiotics as a possible cure so they wouldn't have to add a new medicine, they could always add a new type of experimental medicine found in some sort of event/medical areas... One of the survivors could be a scientist that developed a cure or accidentally released a new strain of virus. The main thing I was spit balling was the ability to play as a zombie/infected. It would make the AI more scary to deal with, because the infected/zombies as they are right now are just pests, not really a threat.

Give option to cut up zombie to produce zombie flesh. Each piece of zombie meat you eat, accelerates the rate in which you turn into a zombie. To help prevent players from abusing this and feeding unwilling players it'll take eating several pieces in order to turn into a zombie. As a player you could cure this sickness with charcoal/Tetracycline, if you have it on your character at the time of turning into a zombie, you can use it to cure your infected state, you can also search for the cure after you turn. As a zombie you'll keep all your gear, however will be unable to use your weapons. As a zombie you have an increased efficiency in knocking doors in, you can crawl through house windows, and you can rally other zombies around you with a yell. In order to survive as a zombie, you'll need to eat player flesh. Maybe could give player zombies a higher chance to cause bleed, and or maybe add special items that they can use like claws, and or getting irradiated and being able to puke radiation causing sickness to the players in the vicinity. Could give them decreased fall damage and good night vision. Adding this to the game will make zombies more terrifying and adding a new type of dynamic game play, will make it more of a zombie survival game. The zombies as they are feel more like an after thought when playing, you just corral them into a house and close the door...