Mutations

"Each week one new Mutation is featured and the previous Mutation retired."

- Valve

Mutations is the name for a new set of gameplay modes scheduled to appear alongside the new campaign, The Passing in Left 4 Dead 2. The Mutations are special game modes that run for one week before it is retired, and replaced by a new Mutation each week as they become announced on the in-game Blog Post every Thursday. Presumably after all the Mutations have been revealed, a poll will be conducted once a month for players to vote for their favorite Mutation, which will return the following week. It is only available to those who have downloaded The Passing.

Revealed Mutations
There are over 20 different Mutations planned, and playable on all Left 4 Dead 2 Campaigns.

So far, the following have been revealed on Valve's blog in the scheduled order along with their taglines:

Realism Versus
As Survivors

In general, as Survivors, you're going to be in a heap of trouble. If possible, try to plan out your route through the level before you leave the safe house. The fewer decisions you have to make on the fly, the better you will do. Additional safe house prep - take a melee weapon if you can. The pistols do you little to no good.

Once you are on the move, keep moving. If a teammate goes down behind you, don't stop. If they go down in front of you, only stop if there is a relative lull. You can't afford to lose a second player trying to save the first. This game will invariably come down to who is the fastest, not necessarily who is the better team. Stay together, move quick, pray.

Using microphones may be a good idea, as even Valve suggested in the in-game Blog Post for Survivors of Realism Versus to use them to communicate faster.

As Infected

Know going in that you have an immense advantage, especially if all players are human. You can see where everything is, humans, each other, etc, while the Survivors you are attacking can only see items.

Even more so than in a typical versus game, separating the Survivors is key. To this end, the Jockey is your most potent weapon. Hop on the back of someone near the back of the group, and lead them back towards the beginning of the level, away from their teammates. By the time the rest of the team notices, it'll be too late.

The Smoker also finds itself more useful than it typically is. Its ability to lurk in tough-to-shoot areas and its impressive range make it a boon for separating anyone from the pack. When used in tandem with a good Charger or Jockey, a team can be effectively crippled.

The Charger has the most potential for devastation in Realism Versus, but remains difficult to use effectively. If you, as a Charger, find yourself in a Scattering Ram-type situation, you can, with little outside help, severely hobble a team, particularly if they have already been hurt before you get to them. Not only will you be carrying and pounding one team member, your hit-stun will both do considerable damage to other survivors and render them useless long enough for you to take out the Survivor you are bashing into the ground. On top of that, they will be scattered in different directions and disoriented, leaving them primed for your fellow Special Infected to pounce.

The Hunter remains the soldier of the Infected team. He's sneaky, he's rangy, he can separate and do serious damage. Fortunately, his gameplay doesn't change much at all here. It's still jump high, pounce hard. It's possible, regardless of whether or not your teammates are good or bad, to take out an entire team as a hunter, if you play smart. Be crafty, and keep picking off the back person in the group. If you get too far away from the Survivors after killing one off, don't be afraid to hit X to jump forward -- due to frequent snarl-ups, you can almost always find a place to respawn nearby.

The Boomer and Spitter are very tough to use effectively in Realism Versus. As the Survivors are typically moving as fast as they can, you almost invariably have to be stationed in front of them. Unfortunately, this isn't very effective, because you don't often know how far ahead of them to be. Your best bet, with either, is to identify an early choke point, someplace that is the only route the Survivors can take. If you are the Boomer, lurk just past it, and either vomit as they pass, or run in and let them kill you in their midst. If you are the Spitter, find a relatively safe spot and keep your sight trained on where you know they need to walk by. Optimally, you will either catch them all or splinter the group in to two different parts, thereby making your teammates work easier.

If you happen to get a Tank (no guarantees there), be cautious you don't lose control of it. If the Survivor team realizes it is a "run for your lives" situation, don't pursue the leader. If you do, and you manage to kill or incapacitate him, you will almost certainly lose control of the tank, and that is worse than letting one guy get a bit of a head start. Your spawning teammates will take care of any front runners, the tank needs to focus on the group or the stragglers.