One shot: They might as well be un-dead, if the rain comes

Into each (reanimated) life, some rain must fall: Valve show off the first look at "Swamp Fever", the Left 4 Dead 2 campaign being unveiled at San Diego's Comic Con, happening later this month.

Presumably, the point they're trying to hammer home is the claustrophobic environmental effects that inclement weather will bring, and/or that the horde really don't like getting wet.