Is Counter-Strike: Global Offensive a Left 4 Dead prequel?

Well, maybe.

Here are both locations, as shot by the keen-eyed rofulz.

That's close enough to be a deliberate reference, yet different enough to not have been a blind copy and paste job. After all, Valve have a history with plugging their universes into one another: with Half-Life and Portal widely accepted to exist within the same world. The most popular crossover theory in this case suggests that the counter-terrorists of of CS:GO are fighting to defuse the biological weapons that spark off the infection of the Left 4 Dead games.

That said, plot is so thin on the ground in both that you can pretty much draw any conclusion you like. One redditor points out that Left 4 Dead's Louis actually mentions Counter-Strike in his dialogue ("man, this is just like Counter-Strike!"), which would threaten the theory somewhat, had another canny redditor not insisted: "There can be a game called Counter-Strike in the Counter-Strike universe. Marvel Comics exists in the Marvel Comics universe."

Inception.

Further examples of Left 4 Dead maps finding their way into CS:GO, with jazzy synth-funk accompaniment.