Mutations

Mutations is the name for a new set of game 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 every Thursday. A poll is also conducted once a month for players to vote for their favorite Mutation, which will return the following week.

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:

For example: For one week, Mutations may feature Realism Versus (presumably combining the two modes for a more challenging experience Survivor-side in Versus Mode), and the next week it could be Chainsaw Massacre; a game mode giving all Survivors a Chainsaw with unlimited gas. It is only available to those who have downloaded The Passing.
 * 'Realism Versus '(Versus Mode)
 * 'Chainsaw Massacre '(Campaign Mode)

