"It's illogical." Sherlock had said, and John had believed him, turning a blind eye to the reporters telling him otherwise.
How he wished he hadn't.
"You're a doctor, John." Sherlock had scolded. "Certainly you of all people would know the impossibility of the claims." Again, he had had a point.
If only he hadn't.
"Mass hysteria. Classic case." Sherlock was always calm in the face of uncertainty.
Maybe if he'd been unsure...
But the duo was the last to believe, and only after it was too late. Now, John had to watch, alone, from the window of 221B as London burned itself to the ground.
If only Sherlock would shut up now.
If only John could think.
If only there was another way...
...
The two dashed through the streets of London, hot on the case of a killer that had caused the panic across the city, and scared even Lestrade and the Yard into quiet complacency with the new regulations, precautions, and evacuation orders.
Sherlock, as always, refused to be moved. And his doctor was always right on his heels.
"John, we should catch him any moment now, and put an end to this 'zombie' nonsense for good. Really, people can be so stupid. Our guy's not even really a killer, just a maniac with a tendency to bite. Mass hysteria ensues, and copycats run rampant. I've said it before, and I'll say it again." Sherlock was still on a rant about their suspect. He'd had enough of this needless panic.
"Hopefully we can prove it and go back to our lives." John believed in Sherlock Holmes, and he would follow him to their deaths. And that's exactly where the detective's god complex was taking them.
"There!" Sherlock sprinted across the street and at their man, slowly inching and limping his way down the sidewalk. He grabbed his arm and turned him around. "Police!"
"Kinda."
"You're under arrest for the assault of Jeffery Lang and-" He was cut off when the man began to snarl and grabbed at his throat.
John pulled his gun and pointed it at the head of their suspect. "Let him go!" Sherlock, trying to pull away, stumbled backwards and fell onto the pavement, and into the light of a street lamp.
"Jesus!" Sherlock panicked for the first time in a long time when he saw his attacker full-on. The man was ashen grey and his nose and the whole right side of his jaw was missing. He landed a kick into the stomach of the man, and tried to roll away, his arm still caught in the grip of the... he had to admit it now. He was caught in the starving grip of a zombie.
John fired his shot as soon as Sherlock was clear, and the pavement was splattered with black ooze and bits of grey-green brain matter. "Are you OK, Sherlock?"
"It's real, John. We need to get back to the flat now." He didn't wait for a reply, only grabbed John's hand and dragged him along and back to flat in record time.
"God, Sherlock!" John finally wheezed out once the door of the Baker Street property was closed behind them. "I thought... What was that?"
"What do you think?!" Sherlock was completely out of character, yelling, pacing a rut in the floor of the entryway, eyes darting about, and biting his thumbnail.
John straightened up suddenly in realization. "Sherlock." He held a hand out. "Let me see your arm."
...
More and more people got bit everyday since John's incident. London was spiraling into sheer madness, and was finally sealed off the rest of the world just yesterday. Three weeks is a terrifying amount time to be all alone in an apocalypse. Well, at least a terrifying amount of time to be the only living being in your flat.
Most of the time, Sherlock just stood in the middle of his room, jaw slack, eyes blank and fixed at some unknown point on the floor. But sometimes, late at night, John heard him growling and scratching long, jagged lines into the wood of the door. He would let out a feral scream, that chilled John to his very core, before becoming silent and unmoving once again.
John's guilt weighed upon him constantly, locking his best friend away to spend the rest of his... well, you couldn't really call it life... the rest of his existence as a caged animal. Everyday, he woke from his restless sleep and tried not to check on his friend through the peep hole he had drilled in the wood. But, everyday, he would give in, and see the most brilliant man in history drooling into his scarf and staring at the floorboards. And, everyday, he sat on the couch, as unmoving as Sherlock, and stared at his gun.
He knew what he had to do, and he had been so close to it every time.
But this was Sherlock he was talking about. The man he had followed to hell and back, and watched die once. He didn't want to be the cause of the second time.
Now, the city he had helped saved was slowly reducing itself to ash and undead, but John couldn't see it through his tears.
Three weeks is a long time to spend alone with a zombie.
Three weeks is a long time to hear that infernal snarling and growling and screaming.
Three weeks is a long time for John to try to convince himself that Sherlock was still in there somewhere.
"But he's not, John H. Watson." He told himself, stepping away from the window. "And you know what you have to do."
He swept his gun up from the table and walked slowly towards the barrier that stood between him and his genius. John opened the door tentatively, stepping inside, and shutting it with a soft click. He leaned against it and took a deep breath. Sherlock swayed slightly as he stood staring at the floor.
"Oh, Sherlock." John whispered, trying not to lose his nerve.
The monster in front of him snapped his head up, catching John's scent and hearing his words. Barring his teeth, he rushed forward, barely registering the click of the safety on a gun.
...
Four weeks is a long time.
If only there wasn't so much time ahead.
But time is no more, and weeks mean nothing to the undead.
Most of the time, John and Sherlock stood in the middle of the room, jaws slack, swaying slightly and staring at a spot on the floor.
But sometimes, late at night, they growled and scratched long, jagged lines down the wood of the door.