Hey Abyssus, I just had a funny idea regarding a multiplayer "infected" dynamic.
Suppose you go for some kind of "crazy infected" kind of zombie. Wouldn't it be fun to model the actual players getting infected! To simulate them becoming crazed, you could change the models/mesh of other players to zombies on an infected players client so to them the other players look like zombies (and maybe zombies to look like normal humans), and consequently they start attacking other players!
The other players are like, woah dude why are you attacking me! LOL
You could do this on a sliding scale over time, so that sometimes the models flick over to zombie, and sometimes not giving the infected player the time to cure the infection before it overcomes them (and they turn into a zombie NPC).