I'm looking for a fic that was first published around the time season 2 aired, and I know updated once since then. It was originally posted on FFN and then later ported to AO3.
It's an AU where the zombie apocalypse has/is occurring, and John is a zombie. However, he is coherent for several days after eating if he doesn't exert himself. The fic opens with John coming back to his senses, and noticing that there is a person running away from a horde (Sherlock, who he's never met) and he decides to draw the zombies away to save Sherlock, despite the fact that would cut into his coherent time. He wanders around, finds an empty apartment and cleans up, and Sherlock tracks him down.
Sherlock doesn't realize he is a zombie until more than halfway through the first chapter, I think, and the chapters were very long. We learn that Mrs. Hudson is alive, and lives with Sherlock in 221B. Sherlock is curious about curing the zombies at the moment to put off boredom. Molly is still at Barts, and provides him with tissue samples.
Throughout what was written in the first 2 chapters, Sherlock starts to experiment on John once he figures out he is a zombie, and is able to extend his coherent period by feeding John his blood. At some point in the second chapter, they go to the Diogenes Club, where Lestrade and Mycroft are (and presumably a large number of other societal remnants remain) to do more research on John.
I remember it being fairly long and well written, but I forgot to bookmark it. I would be very grateful if anyone recognizes it.
community has yet to find it in several weeks, so I hope to have more luck here!
(For tags....perhaps post-apocalyptic or zombie would be useful tags?)