John pushed a £20 note into the taxi driver's hand before jumping out of the cab and rushing to catch up with Sherlock as his walked quickly towards the latest crime scene.

This was the latest in a string of murderers that had so far claimed five victims, and when all of their efforts had failed to leas them any closer to the killer, Lestrade had finally come to Sherlock for help. Sherlock had seemed almost obscenely excited when he had been invited to the crime scene, but he had rapidly sobered in the cab on the way to the scene, and now he was bordering on sullen.

The latest crime scene appeared to be exactly the same as the reports of the previous four had described. The victim was a young woman, lying naked on the bed, her wrists slashed, the bed sheets soaked with blood.

Sherlock stepped slowly into the room, his eyes falling on everything as he rapidly absorbed all of the facts he could see. Finally, after standing slowly just in front of the door for a moment, he strode up to the body, and began to examine it.

"Well?" Lestrade asked, stepping into the room after John. "Any thoughts?"

Sherlock stood up straight, a look of confused frustration on his face. "I don't know." He said simply. "I can't..."

"Sherlock?" Lestrade prodded quietly. "Are you alright?"

There was a brief moment of tense silence, in which Sherlock's eyes drifted shut, before he looked up at Lestrade. "No." He said simply, and then, as if his legs had disappeared from underneath him, he collapsed.

"Shit." Lestrade darted forwards and fell to his knees beside Sherlock. "Anderson! Donovan!"

Almost immediately, the two officers appeared in the room.

"Oh Christ." Anderson muttered when his eyes fell on Sherlock.

"Anderson, get the kit." Lestrade barked, reaching into his coat pocket, pulling out his wallet and throwing it to Donovan while Anderson raced out of the room. "Donovan, there's a Tesco Metro round the corner. Go get the usual."

Donovan nodded and ran out of the room just as Anderson returned with plain plastic tub and tossed it to Lestrade. John could only gape, confused, at the activities going on around him. Sherlock had just collapsed for no apparent reason, and the best of Scotland Yard were acting as though this was completely normal – expected, even. They were like a well-oiled machine, moving around each other in a perfect routine, alarmingly well-practised.

Lestrade pulled a small black case out of the plastic tub, and immediately grabbed Sherlock's hand.

The detective struggled sluggishly to pull his hand away, batting feebly in Lestrade's general direction with the other, mumbling what sounded oddly like 'Sally'.

"Yeah, she's gone." Lestrade said, getting a firmer grip on Sherlock's hand and jabbing him with what John recognised as a finger pricker from a blood testing kit. "She'll be back in a minute and we'll have you back on your feet calling us all idiots in no time."

In one swift move, he dropped the pricker back into the box, grabbed the small blood testing machine, and held the end of the plastic strip against the drop of Sherlock's blood on the end of his finger.

"0.9." He read out a moment later, throwing the machine into the box next to the pricker. "Double shit."

Lestrade immediately grabbed a small, plastic orange box from the kit, and opened it to pull out a syringe and a small glass vial of white powder – glucagon, John's medical mind supplied. He quickly flipped the cap off the vial and inserted the syringe, pushing the plunger to squirt the liquid from the syringe into the vial.

"How low is he?" Donovan asked as she walked back into the room, dropping a small bottle of sports drink, a packet of chocolate biscuits and a chicken sandwich on the floor next to Lestrade.

"0.9" Lestrade told her, shaking the vial to mix the solution and reinserting the syringe to draw up the clear liquid. "Don't know how he managed to stay on his feet this long."

Nodding in satisfaction, Lestrade withdrew the syringe from the vial, and gently flicked the side before pushing the plunger slightly to remove any bubbles. Then, in one smooth manoeuvre, he flicked the buttons of Sherlock's coat open, shoving the thick material aside, pushed up the front of his coat, and jabbed the inch-long needle into the flesh to the left of the detective's navel, pushing the plunger right down to inject all of the liquid.

"Right then." Lestrade said, withdrawing the needle and putting the cap back on, sitting back. "Let's give that a few minutes."

"Sorry." John said, staring open-mouthed. "What's going on? Did you say his blood sugar's 0.9?"

"Yeah." Lestrade replied. "Looks like the bloody idiot hasn't been eating again."

"He ate last night." John told them. "Chicken chow mein. But his blood sugar shouldn't have dropped that low. His own body would have kicked in first."

"Yeah." Lestrade agreed. "If his body worked. Come on John, you're a doctor."

"What do you mean, 'if his body worked'?" John asked, looking baffled.

"Well he's diabetic, isn't he." Lestrade said, staring at John.

"I'm sorry." John said again. "He's what?"

"You didn't know he's diabetic?" Anderson said, staring.

"No." John told him, glaring at Sherlock's unconscious form. "He never mentioned that to me. But... hold on. He was fine. A bit moody when we got here, but he's always a bit moody."

"Hypo unawareness." Donovan chipped in, crouching down next to Lestrade.

John gaped. He had heard of hypoglycaemia unawareness, of course – those cases where a diabetic lost their hypo symptoms, but he had never actually encountered it. He opened his mouth to say something – although he honestly had no idea what he was thinking of saying – when, suddenly, Sherlock started shaking, his body twitching in small convulsions, his head banging against the wooden floor and his feet scrambling against the floorboards, as if he was trying to pull his body away from something.

"Here we go." Lestrade said, leaning forward and gently sweeping Sherlock's sweat-soaked hair away from his eyes. "John, stick your coat under his head."

John nodded, pulling his coat off and dropping down behind Sherlock to gently lift his head and rest it on his folded coat.

"Sherlock?" Lestrade called, continuing to stroke the detective's head. "Sherlock, can you hear me?"

"Mmm." Sherlock affirmed, rolling slightly in an attempt to bury his face in John's coat.

"No, Sherlock." Lestrade said. "You can't go to sleep."

"Mpf!" Sherlock grunted irritably. "Mm... wanna."

"I know you want to sleep." Lestrade said, tapping Sherlock's face lightly. "But you need to sit up and have some food."

"Can't." Sherlock mumbled, pressing his face further into the coat. "F'koff."

"Charming." Lestrade laughed. "I'll remember that next time I have an interesting case I think you might like. I'll just tell myself 'last time we had a good case, Sherlock told me to fuck off."

"Fuck." Sherlock said. "Off."

"No." Lestrade told him bluntly. "Now, John's behind you. I'm going to get Anderson and Donovan to lift you to sit up against him, ok?"

"Don't care." Sherlock grumbled, shaking his head petulantly. "Don't like."

Lestrade nodded to Anderson and Donovan, and they both stepped forward to grasp Sherlock under the armpits, lifting him and placing him gently into John's lap, his back resting against John's chest.

"Now." Lestrade said, grabbing the drink and biscuits. "How about you eat a biscuit while Anderson calls us an ambulance, eh?"

Sherlock opened his eyes slowly to glare at Lestrade while Anderson quickly pulled his phone from his pocket and dialled 999.

"Don't. Want." He snapped, turning his face to hide in John's chest. John looked helplessly at Donovan, watching as she slipping a straw into the bottle of Lucozade, cringing in sympathy as Sherlock gave a small, feeble little whimper, and what sounded worryingly like a sob.

"I know, Sherlock." Lestrade said sympathetically, turning Sherlock's face towards him and holding a biscuit to his lips. "But you'll feel better if you eat these, I promise. Bourbon creams. Your favourites."

Sherlock glared and opened his mouth, allowing Lestrade to push the biscuit inside, before snapping his mouth closed, smirking as he caught the tips of Lestrade's fingers between his teeth.

"He's definitely getting better." Lestrade laughed. "Blimey, your teeth! Have you got space teeth?"

Donovan snorted, leaning forward to hold the straw to Sherlock's lips when he coughed around the dry biscuit. Sherlock gulped the Lucozade down eagerly, crinkling his nose and pulling his face away when he had had enough of the sickly, sweet drink. He nodded gratefully to Donovan as he held out his hand towards Lestrade for another biscuit, before pushing it grudgingly into his mouth with a mumbled "Thank you, Sally."

Lestrade nodded, leaning back and grabbing the blood testing kit. He quickly and efficiently tested Sherlock's blood sugar again, smiling as he held the result up for Sherlock to see.

"2.9." He said triumphantly. "You're on the mend."

Sherlock nodded, accepting the straw from Donovan for another mouthful of drink as the sound of heavy footsteps reached them.

John looked up and saw two paramedics entering the room.

"Bloody hell." The first said, his eyes falling immediately on the body on the bed. He rushed towards her, but Anderson grabbed his arm to stop him.

"Ignore the body." He said simply. "The patient's this way."

The paramedics stared. "But..." One said, staring from Sherlock to the dead girl and back again. "The girl..."

"She's been dead a while." Lestrade told him, reaching into his pocket with one hand while he handed Sherlock another biscuit with the other and holding out his police identification. "We were just investigating the murder when our consultant here collapsed."

"Right..." The paramedic nodded, looking uneasily at the body on the bed before kneeling down next to Lestrade and focusing on Sherlock. "What've we got here then?"

"He's a type one diabetic since he was ten, and he's had problems with hypo unawareness for about eighteen months." Lestrade told him. "Collapsed shortly after arriving on the scene with a blood sugar of 0.9. We gave him glucagon, and now we're working our way through the biccies and Lucozade."

"Brilliant." The second paramedic said, rooting around in his bag. "Have you done a blood reading since?"

"It was 2.9 about five minutes ago." Sherlock said around a mouthful of biscuit, wincing as his finger was pricked again. "I should imagine it's about four or five now, and I can go back to the dead prostitute on the bed."

"Not yet, you won't." Lestrade told him. "The last thing we need is our murderer claiming you had lowered brain func... wait... Prostitute? What?"

"Yes, Lestrade, prostitute." Sherlock said, nodding as the paramedic held up the blood testing monitor to show the result of 4.3. "She is living in a run down, dirty block of flats, and yet can still afford to buy heroin."

"That doesn't mean she's a prostitute." Lestrade argued, glancing back at the body. "She could just be another junkie living in squalor to fund her drug habit – no offence."

"I no longer live in squalor to fund my drug habit." Sherlock commented. "Why would I be offended. Anyway, there is also a box under the bed containing a variety of expensive sex toys – a long whip, a riding crop, four pairs of handcuffs, two gags, a blindfold, and six vibrators, I believe. She was also in her own flat at two o'clock in the morning wearing a considerable amount of frankly quite unsubtle make-up. Now, she is a blonde, excessively well-groomed woman, and yet there is a dark brown pubic hair on her inner left thigh. Run that through your databases, and you'll have your killer. No doubt he's been arrested for at least kerb-crawling at some point." He smirked, grabbing the chicken sandwich as Anderson held it out for him. "You lot must be even more brainless than usual, today. I figured all that out with a blood sugar below one."

John sighed, watching Sherlock carefully as they stepped out of the building and walked through the police blockades.

"John, please stop staring." Sherlock demanded irritably. "I promise you I'm not going to collapse again." He glanced up and groaned. "Although I think unconsciousness may actually be preferable right now."

John looked up in the direction Sherlock was watching and suppressed a groan of his own as he saw Mycroft approaching, ever-present umbrella and Anthea in tow.

"I can't believe you didn't tell me you're diabetic." He mumbled, glaring as they approached the older man. "Idiot."

"Ah, Sherlock." Mycroft said, studying Sherlock closely. "I hear you've not been taking care of yourself again."

"My blood sugar was 10.6, so I took a corrective dose of two units of insulin." Sherlock drawled. "That should have lowered my blood sugar to a perfect level of around five. It's not my fault your damn government make it so difficult to get an insulin pump when you need one."

"Yes, well..." Mycroft smiled tightly, his eyes dancing between Sherlock and John. "Now, did I read Doctor Watson's lips correctly? You failed to inform him of your condition?"

Sherlock groaned. "You're not allowed to speak at all when Mycroft's around in future." He muttered to John, before turning back to Mycroft. "Yes, fine. You've made your point. John should have been informed – although I'm surprised you hadn't already made that decision for me – and I am to monitor my levels more closely in future. Are we done now?"

He stormed past Mycroft without waiting for a response, his coat billowing behind him.

"Keep an eye on him, Doctor Watson." Mycroft said as John went to follow. "He would get the pump, if only he'd go to the necessary training courses. Do try to get him to go, or at least to look after himself."

"I will." John nodded, smiling stiffly at Mycroft, before turning and following Sherlock away from the scene.