Gradually, a poll was conducted for players to vote for their preferred Mutation, which returned the following week as an Encore Mutation, which means it might re-appear the next week . With the download of The Passing DLC it's possible to choose what Mutation you get to perform. All mutations were made available for play at any time on 07/24/
In 2010, after every four or five weeks, Valve would announce an encore week where the community vote for a Mutation to be replayed. Recent Mutations which were played can be voted for. Otherwise, the developers would choose to replay one of the preferred Mutations for a week as a 'Developer's Choice'. The following encore weeks re-showcased the following Mutations:
Mutations is the name for a set of modified gameplay modes, the first of which was released together with the campaign, The Passing in Left 4 Dead The Mutations are special game modes that were available for one week each before being retired, and replaced with a Mutation per week following an announcement on the in-game Blog Post every Friday.
From Valve comes Left 4 Dead, a co-op action horror game for the PC and Xbox 360 that casts up to four players in an epic battle for survival against swarming zombie hordes and terrifying mutant monsters.
As InfectedGoing in, realize that you have an immense benefit, particularly if all players are human, however for competitive play that you still want to disable the enemy team as quickly as you can give yourself an easier time as Survivors.
These were found at a Steam-Group announcement and have never been seen in a schedule as of yet. They may also be found amongst the script files of the game. They may still be subject to removal or changes, as Valve has not announced them yet.