S2, E6: Epidemiology

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Pierce and a few other Greendale students ingest a biohazard substance at the school Halloween Party, causing them to exhibit flu-like symptoms and begin turning into zombies. It is up to the rest of the gang to save themselves and the school when Dean Pelton locks them in with the zombie-infected student body.
Starring George Takei, Jim Rash, Dino Stamatopoulos