There is no cure. They both know this, have known this since they first saw the reports of people coming back from the dead, springing up from morgue tables and ripping out the vocal cords of the first victim they could clutch in their stiff fingers. Sherlock locked himself in a fortified room at Barts with John armed at the door for twelve hours and searched every virus database he could find, got on the phone with every health department he could beg to listen and … nothing.
It seemed as if this disease had just emerged out of nowhere in isolated populations across the entire world. There are reports of it in Fiji and Finland, in Japan and Java. Nowhere is safe from the return of the departed; corpses as old as two weeks are beating their ways out of refrigerators and attacking anything that moves for apparently no reason than to spread the disease.
With more time and more data he could probably figure something out, whip up some magical concoction to inoculate themselves and any survivors they could find, but all the hospitals are on lockdown, the government has issued a blackout and a 24-hour curfew, and they are trapped in Baker Street with the windows boarded up, the doors covered with the heaviest furniture they could manage, and the mutilated remains of Mrs. Hudson mouldering a level below them.
John doesn't want to have to remember pumping a full barrel of bullets into a bloated, putrescent Molly, who nearly decapitated him the last time he'd escorted Sherlock to Barts to pilfer more equipment. That was the last straw, and now they remain firmly ensconced in their quarters, dead silent so as not to attract attention from anything waiting for them outside.
If Sherlock looks pale and glossy with sweat, John imagines that it's just the stress of their situation. Having to destroy some of their closest friends is difficult for him, a trained military man, and he can't imagine the trauma it has enacted on his friend, who has no experience in losing anything, much less the only people he chose to care about. He even rationalizes it away when Sherlock begins to exude the peculiar sickly-sweet odor of rot rumored to appear in the infected; after all, they have not had the chance to shower in several weeks, and he doesn't smell like roses either. They cannot speak for fear of revealing themselves, so he babbles comfort to himself in his head, knowing in the deepest part of him that Sherlock is soon to leave him, too.
Finally the detective comes to him in the living room, where he is trying to take his mind off apocalypse by re-reading a favorite book. His face is stony but his gait is steady and his words unslurred, as if he's been practicing for this moment.
"You should leave. Go see if Harry is still alive. People in the country might have survived."
"Don't fool yourself. Anyone without military training or self-defense expertise was probably infected within a few days, and even professionals likely got caught up in it too." John refuses to look at him. "I know what you're trying to do, Sherlock. But I don't want to leave. Not without you."
He doesn't have to see his flatmate's face to know his expression, the way his eyes are crinkling with anger as he spits out, "I'm infected, John. I've been trying to inoculate myself, I've tried every single fucking potion I could come up with, and it's no use. It's either you leave, or you get infected too. And I don't want to have to – I –" The words choke in his throat, and he takes a hitching breath. "I don't know how much consciousness the deceased maintain. And I don't want to even consider the thought that I might … the things I might do."
John puts the book aside slowly and stands up, squaring his shoulders. He looks his best friend in the eyes and sees the dark violet ring emerging around his irises. Soon enough he will lose all semblance of humanity, will become a violent psychopathic murderer just like the corpses shambling around outside. The thought makes him nauseous with rage, that a bacteriophage is about to take away the greatest mind the world has known, and there's nothing he can do about it. "You don't want to become one of them, do you?"
"Obviously not. No."
He smiles sadly and reaches for the cold, clammy hand of the detective, engulfing it in his right. It feels almost like a wedding ceremony, a strange joining of hands here in their desecrated living room while the living dead scream and thrash on the blood-soaked stones of Baker Street.
John pulls out his service weapon from the waistband of his jeans. He never goes anywhere without it anymore, doesn't take it off his body even to sleep. Still clutching the hand of his friend, he stands beside him; they both face the mantle, with its skull and jack knife and bundles of papers and the memories, all the wonderful memories that burden it with their dust and tears and dreams.
The doctor puts the gun to his temple, squeezing the fingers of the only man he has ever loved. His hands do not shake and he keeps firm pressure on the trigger, angling it for the best impact through both their skulls. "It was good, yeah?"
"Yeah." Sherlock leans the left side of his head against the right side of John's, closing his eyes.
John Watson and Sherlock Holmes leave the world not with a whimper, but a bang.