' Bacteria' is a name for the Special Infected jingle that plays when one is around.
This usage of an often-repeated cue in Left 4 Dead 's soundtrack is similar to how some real-life movies have cues (or leitmotifs) which recur throughout their soundtracks. The noticeable cue in this song reoccurs in many other musical pieces in the game, such as in the Tank's themes. ' Left 4 Dead' is the main theme of the game.
