Cold Stream sees the Left 4 Dead 2 survivors fighting through a forest in the mountains to reach a helicopter to evacuate them before a forest fire catches up with them, while The Last Stand represents an alternate interpretation of what had happened in Death Toll had the survivors gone a different route.
Having now finished the original two Left 4 Dead campaigns, the only thing that was Cold Stream and The Last Stand, two community missions that rounded out the game.
“I need a weapon.” –Spartan John-117, Halo 2