Swamp Fever

Swamp Fever is a new campaign which takes place in a "swampy" location in Left 4 Dead 2, according to Valve the Survivors will reach this swamp after evacuating from Savannah, it is believed to be the third (most likely) or fourth campaign. It will be first revealed to the public in ComicCon 2009 through 23rd July to 26th July and will be fully playable to the public views in the Left 4 Dead Demo which will take place near November 2009. It seems as though it's a showcase for the AI Director's new, more in-depth level controls—such as the weather, and the new water system which slows the Survivors down and does not allow them to run.

Trivia

 * Swamp fever is a term given to a number of diseases that are acquired in wet, swampy environments.