Left 4 Dead was released by Valve in 2008 as a co-operative first-person shooter about four survivors fighting infected hordes. Its dynamic music system changed intensity with the action, and the in danger sting became one of its most recognisable cues because it meant a teammate was down and the team had seconds to react. Clips and edits shared online later reused the sound outside the game, where its frantic strings still instantly signal trouble, similar to a danger alarm sound effect meme in other memes.