Disclaimer: nothing you recognize is mine. Obviously.
Chapter 1
When after such a conversation he develops a persistent, pulsing, annoying (if not severe) headache, a lesser man than Sherlock would think he jinxed himself. Instead he just goes to sleep. After all, it's just after a case and he deserves it.
The following day, his worry isn't better, but he ignores it steadily. He's decided it might be a consequence of opening that specific bolted room on his mind palace (John doesn't have monopoly on psychosomatic ailments) and the best thing to do is busying himself with something – anything. Now, if only he had anything to do...John's patience is sorely tried that day, but being bearable is the last of the detective's concerns.
By night, Sherlock is frayed enough that he pilfers a paracetamol from his flatmate, even with little faith in its effectiveness. Since sleeping did nothing for his head, he doesn't. Better to be as normal as he can. He takes up the violin instead, choosing a soothing melody. He doesn't want to purposefully irritate anyone, so he might as well play something likely to merge seamlessly into people's dreams. At 3:24 am, though, a jarring note disturbs the darkness. He made an honest mistake. It doesn't happen since he was 13. By the look Sherlock gives his beloved instrument, you'd think he's just been bitten by it. Gently still – that's too ingrained in him by now – he puts it back.
He doesn't appear at all after that, and at lunch John knocks and then quietly allows himself in, when no answer comes. The doctor finds his friend on the bed, curled into the tightest, littlest ball of misery he's ever seen an adult man (never mind one as tall as Sherlock) achieve.
"Sherlock? What's wrong now?" he asks softly.
Nothing suggests he's been heard.
"Either you answer me or I'm bringing you a plate of something. Of my choice," the doctor says, wondering quietly how food between them is used as a threat instead of a bribe.
"I missed a note," his friend grinds out finally.
"That's it?" John replies, because it seems...excessive. Even for Sherlock.
"You don't understand, John!" the detective yells, uncurling to throw himself towards the doctor. Sherlock's right, of course. "I don't remember the whole musical score of the arrangement for violin of Chopin's nocturne n. 20, short as it is. And I didn't delete it." He whispers, now, eyes wide with undeniable fear.
"Sherlock, calm down now. Stress doesn't help you to remember anything. It could be a momentary problem. You know, like normal human beings happen to have," John prompts, voice quiet and – hopefully – relaxing. Really, that's ridiculous.
"I've the palace in place not to incur in these normal humans' occurrences, John! I can't allow to have them!" the detective growls, glaring harshly.
"Though luck, because you're human, Sherlock. Thankfully," the doctor states in his best no-nonsense tone.
You don't know, John!" the sleuth protests, almost plaintive.
"So tell me!" his friend half-requests, half-pleads. It makes no sense. Sherlock doesn't fret as much over missing details on a case, for crying out loud!
"I won't!"
Sherlock takes his phone instead, and an irritated John is about to leave him alone, when a quick gesture stops him on his tracks.
The sleuth actually calls, not texts, and it's enough to make John worry. When at the second ring someone answers, and Sherlock replies softly, "Mycroft," the doctor knows it's serious and almost shivers. Sherlock calling Mycroft willingly? The older brother is informed of the 'emergence', with a lot of, "It's unfair!" and angry, "Why are you okay?", and John expects him to offer a firm lecture over what constitutes an actual emergency, but Sherlock's face is not of one being reprimanded.
"Will you tell John?" the detective asks, before handing over the phone.
"I'm sure you reckon Sherlock is being over-dramatic as usual, but he has an actual reason for his distress," Mycroft assures him calmly, which is very distressing to John in turn, but he tries to keep it under control.
"What?"
"What do you know of EOFAD, doctor?" Mycroft counters.
"I might know something if you give me the non-shortened version; now I can only say a bullet doesn't cause it," he replies.
"I'd be very surprised if a bullet triggered an autosomal dominant disease like Early-Onset Familial Alzheimer, Dr. Watson. This ailment tormented our father, unfortunately...so you see why Sherlock and I might be oversensitive about our memory's failures, even the most insignificant," comes the cold, collected voice over the line.
"Dominant means he'd be sick even if half his genes didn't carry the defect responsible for it. With enough luck, you both could not have inherited those. Have you been tested for that?" John asks, in full doctor mode.
"It's bad enough being a ticking bomb, John...neither of us dared to know if we would really go off someday. Sherlock's dislike of hospitals didn't help."
"Well, that's silly; you both should. Or at the very least Sherlock, because having a panic attack over the tiniest memory slip isn't healthy," John remarks. It could give them peace; or not, but they'd know to prepare for when it happens.
"I'm not going to," the detective grumbles.
"Thank you Mycroft, I'll let you know," the doctor says before hanging up. "I'll come with you, Sherlock, come on."
"And hold my hand? I'm not a child, John!" the sleuth snarls.
"You could be healthy, Sherlock!" John shouts. He'll regret that.