What about an infected mechanic?

Who is infected and when is kept secret but once you are are infected you have so many rounds before you turn. Once infected you get two options search for a cure gaining bonus points and immunity in the future from infection. Or you can try and infect other players. and start sabotaging the team getting a bonus for each person you cause to die.

is this an online board game? The answer to that question hugely impacts your choices when it comes to such factors as alternate goals within a game.

In an online environment, a full view of the game cannot be iterated, thereby allowing the implementation of any number of secretive tricks. I'll call this one a 'grace period'. This grace period will, to put it simply, create a 10-15s window after a player makes a move in order to allow the 'necromancer' to counteract that move or make one of its own. For example, you have 4 players. no-one knows who's turn it is except the person taking it. The necromancer doesn't have a turn, instead he has the grace period where once a player makes a move, he has a 10-15s window where he can play his own move (the necromancer will hold the same rules applied to the players turn, so if the player has a limited amount of actions so will the necromancer).

This would be an interesting factor, as it would lead to dissension within a group. not knowing who's turn it is would, in fact, lead to a lot of suspicion of each member. Sure, they can 'claim' that they are no the necromancer, but you don't know who's turn it is.

A counteract to such an event would be that the person playing could tell everyone his next move, thereby alleviating him of any suspicion. To stop this, you'd have to implement a chat off time, where the chat to discuss strategy's among other things can only be accessed at the end of a turn, not the start.

However, I digress too much. If this is not an online board game, you start to run into troubles. First of all, not many people hold well under pressure. You'll find that quite a lot of the time, the necromancer will reveal himself simply through facial expressions. While this is effectively another tactic within games such as poker, a game that has one of its core components being revealed so easily would cause a lack of entertainment. Another problem is the simple fact that communication has to be implemented whenever a move is made. As the board is known to everyone, any moves will be known as well. So if the necromancer makes a move, they will be found out immediately.

Well, as I reread your comment, I've come to the realisation that much of what I've said is effectively irrelevant. Ah well, perhaps some of what I said will help you. To answer your original question, the fact of the matter is that zombies is simply a term to describe dead who have risen and, in some form or function, live. This subject is a lot more broad then people realise, just as commonly people mistake an infection type to be the only zombies. But I digress, the answer is that next to nothing you pick will dissuade your zombie theme, as zombies are quite broad (as I said). Hell, you could decide that Nikola Tesla was running electricity tests and accidentally created a zombie army through gods knows what reason, and is now undercover within a group of highly skilled scientists who are working on a cure in order to eliminate them one by one.

or be generic and say they're infected. But that's no fun, is it.

Physical board game, not a computer game.

The idea comes from the board game Battlestar Galactica. The secret player ("traitor", in that game an Alien trying to stop the humans to get to Earth with their convoy) try to effect the others by voting with cards. All vote-cards are played in secret so the others will try to find out which players put which cards (cards can help or work against the group depending on how they are played) and thereby find out who is the "traitor".