Originally Posted by Wrexify

You're correct, it IS ridiculously fun! My brother plays L4D in single player and still enjoys it quite a bit. It's not that it doesn't have a single player, it's just whether you choose to play with bots or with other people. It's the same maps, same rules, same gameplay experience, but without the human interaction.

Oh, and I suppose you can't play as the Infected if you're not playing online versus.