I think zombie fic has become its own genre, really, but Glee/zombie crossovers have become a bit of a thing within Glee fandom; authors took that concept and ran with it (there are a LOT of Glee/zombie fics out there; Gleeks love their apocalypses) and that's why I used subgenre. It's kind of like the hurt/comfort fics with Kirk and Spock waaaaay back in the day.

ETA to fix a pesky apostraphe.