While the brilliant show "Community" regularly get critisized for being too 'gimmick'-y, I disagree wholeheartedly in this case. I think it was done in good taste and it left me longing for more!
Here's what happened. Greendale Community College organizes a Halloween party and the Dean (the hilarious Jim Rash dressed like Lady Gaga) got some snacks. While the evening, filled with ABBA, progresses, more and more people start to fall ill. The sick people start out with flu-symptoms but then go on having Zombie-like tendencies and biting the non-infected. Annie has asked Doctor Rich to determine what's going on, while the others try to fight their classmates off. The Dean, in the meantime, had locked the group in the library with the entire flesh-craving studentbody. Everyone fends for themselves as things start to heat up.
So, in conclusion and without spoiling too much: Best. Halloween. Episode. EVER.