The first time it happens, John is at surgery.

He's walking down the hall on his way to examine a ten year old with a possible case of bacterial meningitis when his breath is sucked out of his chest by some invisible vacuum. His vision flickers in and out of clarity for a few moments and he throws an arm out towards the wall, looking for purchase on the handrail he knows is there.

Vertigo sets in after that. He drags himself into an empty room and collapses on a gurney, chest heaving, desperate for air.

The symptoms pass a few minutes later and he is back in the hall, smiling at the pretty nurses. Eventually the entire episode leaves his mind.

That night he and Sherlock chase a criminal across London and back before catching him. They solve the case and save the day, as they always do, and when they return to 221B John is amazed at how tired he is.

God, I'm getting too old for this shit.

A year passes, episodes of dizziness and shortness of breath sprinkled generously throughout the months until, one day, it suddenly gets so much worse.

He's sitting at the kitchen table that morning, frowning at his toast.

"You're not fat, John," Sherlock says. He's standing with a plate of (burnt) eggs in one hand and a cup of coffee (no sugar this time) in the other.

John's head snaps up. "What?"

"I said you're not fat. In fact, you're in prime physical condition which is why you need to stop this ridiculous diet you're on and –

"You think I'm on a diet?"

Sherlock tilts his head to the side and studies John's face. "You've not been eating. It's the only logical deduction."

He gives a snort and a chuckle. "Really, Sherlock, I just haven't been hungry lately."

Sherlock sets the eggs and coffee in front of him. "Eat," he says.

"You don't eat," John points out.

"I'm different. Eating slows me down. You need to eat in order to keep up with me. I need you in top form."

"So this is about you, is it?" His heart speeds up and thuds strangely against his chest. Selfish, unreasonable, pig headed man! "I don't need a nanny, Sherlock!"

"I can't rely on you when you're half dead from hunger."

John pushes away from the table and stands. "Right. Course. Because that's all you ever worry about, isn't it? How something will affect you and fuck all if someone else is involved or feels differently!"

He grabs his coat and heads for the door.

"John, please," Sherlock's voice is soft, like a plea, "Just eat something."

He eats every damn bite of those burnt eggs.

They head out around noon. Sherlock is determined to harass Lestrade into an early grave –"Early. Ha!" – and will therefore stalk the poor man to every crime scene he attends until something interesting pops up.

They arrive in a cab and his flatmate bounds out the door and across the police tape before they've come to a full stop. John pays the driver – making a note of his face because he'll never make that mistake again – and trails behind his friend.

He moves slowly. He's entire body feels much too heavy.

Not enough bloody sleep. Bloody violin at bloody two in the bloody morning.

Sherlock is giving Anderson a tongue lashing. He can tell, because Anderson looks as if someone's jammed a red hot poker up his arse. It's amusing and slightly disturbing.

He shakes his head and searches for Lestrade. He sees him, on the other side of the crime scene, talking to Sally.

He takes two steps in their direction and stumbles.

His heart is beating a wild, ragged tattoo in his ears and he's no longer just short of breath – he can't breathe at all. He tried to focus on Lestrade, but the man is a wibbly gray blob in the distance.

Oh God, Oh God, Oh God…

He opens his mouth but nothing comes out.

The city is swaying, building bending in the breeze like trees in a hurricane. Or…is he the one swaying? His stomach is doing flips and he can't think, can't think, can't think…straight.

Help. Someone help me.

The roar of the city falls away and all he can hear is Sherlock – "John? John!" – as London goes dark and his body pitches forward onto the hard concrete.