The trailer reveals that the episode will act as a prequel for Valve's zombie shooting series, with a heist in Left 4 Dead's Mercy Hospital. "Have you ever wondered how the Left 4 Dead series began?" the YouTube description asks. "It started with a heist!" So by playing this, you'll very likely trigger the zombie pandemic in those two games. Real nice, butterfingers.
Update: Valve's Chet Faliszek has clarified to Kotaku that the YouTube video isn't a prequel, saying the video description was the work of "some over-excited marketing guys." The map is a Overkill mission "simply set in no mercy hospital," he says. It is not part of the Left 4 Dead narrative in any way.