They were on the train home to London when John sneezed.

A minor thing, really, and Sherlock didn't give it a second thought, but continued to analyze his memory of Fear. The effects of Frankland's hallucinogen had really been quite fascinating. The memory of the way the whiskey glass had felt in his sweating hand as he sat by the fireside was, in retrospect, quite intriguing.

He had felt fear before, of course. No matter that others liked to imply that he was a machine without human emotions, he did have them. He just refused to let them affect his rational decisions. Like he had told John months earlier, caring about people would not help him save them. Keeping emotion out of his thought processes made it possible for him to do his job.

Still, Frankland's experiment had provided him with valuable insight into how ordinary people experienced fear.

He glanced at John, who was staring out the window at the passing countryside. Really, it had been impressive, watching him in the lab. Having been so nearly incapacitated by Frankland's Fear himself the night before, watching John run through the lab, exploring his options, had been illuminating. When he couldn't get out of any of the doors, crawling into one of the cages had been the smartest thing he could do. Sherlock had been impressed. It was so easy to forget that John-unassuming, mild-mannered John-had been a soldier. Of course, he had always known John was brave.

John sneezed.

The dust, presumably, was the cause. Train compartments never were as clean as they should be.

Then John sneezed again. And again.

Sherlock looked at him again, this time really observing his hunched shoulders and pale skin. "Are you all right?"

"I'm fine," John said automatically, and then promptly sneezed yet again.

By the time they arrived in London, John's skin was sweating and his voice (not that he spoke much) had deepened by half an octave. The sniffling had been grating on Sherlock's nerves for over an hour.

Thankful to escape the compartment, he leapt off the train and was striding through the station when he realized that John wasn't behind him. Looking back, he saw him shuffling along in a poor imitation of his usual soldier's pace. Impatiently, he waited until John caught up. "What is wrong with you, John?"

"Nothing. I'm fine." John told him, as Sherlock turned and paced toward the street.

John stumbled getting out of the cab at Baker Street, and leaned against the wall while Sherlock pulled his keys out of his pocket. Once inside, Sherlock bounded up the stairs, his mind already thinking ahead to his next experiment. Really, the trip to Dartmoor and Baskerville had refreshed him immensely. He barely noticed that John went straight up to his bedroom. It wasn't until, hours later, he wanted a cup of tea that he realized John hadn't come back down.

Curious. John was usually puttering around being so quaintly domestic, complaining about the chemicals in the kitchen and going on about food contamination. Since they had come home, though … over six hours ago … he hadn't said a word. In fact, he didn't remember seeing him at all, not even tapping his two-finger tap on his laptop.

Sherlock went upstairs and nudged John's door open. His overnight bag was abandoned in the middle of the otherwise neat room and John had climbed into bed without (Sherlock blinked) even taking off his shoes. John didn't even nap on the couch with his shoes on. What was he doing with them in his bed?

Now that he was looking, he realized that John's color, even in the dim light, was much paler than usual, except for a hint of red on his ears and nose. His skin was shiny with perspiration and his breathing was unusually noisy, rasping moistly as he lay there with his mouth open.

"John, are you all right?"

Wearily, John turned his head. "No, Sherlock. I'm not. This is just a happy little souvenir from my time trapped in the Baskerville lab. Thank you SO much for that, by the way." He rolled over in bed, turning his back to Sherlock. "Now if you wouldn't mind, just go away."

Stunned, Sherlock backed away and headed for the stairs, unexpectedly chilled. The Baskerville lab? With all the mutations and dangerous experiments they worked on? What had Dr. Stapleton said? "If you can imagine it, they're doing it?" John hadn't gotten beyond the first level of laboratories, which should have been safe, but what if there had been more than just Frankland's hallucinogen leaking from those pipes? What if there had been something … else?

He shook his head, telling himself that he was worrying over nothing.

Still … he did a quick search and found that John's symptoms could explain anything from a sinus infection to anthrax poisoning.

-What the hell are they doing in Baskerville? SH

-Why do you ask? MH

-I suspect John may have been poisoned or infected at the lab. SH

-Highly unlikely. What makes you think so? MH

-He's ill. SH

-What does he think? He's the doctor. MH

-Very funny, Mycroft. I'm trying to save John's life and you're making jokes.

-Not at all. I simply suggested you ask him what's wrong with him. Good night. MH

Sherlock stared at his phone. That was cold, even for Mycroft. What if John was dying? How could he find a cure if he didn't know the poison?

He heard a noise behind him and spun around just in time to see John walk into the kitchen. He bounded across the room. "You're up! How are you? Are you all right?"

John was peering into the refrigerator past the container of ears. "I'll be fine, Sherlock. I was just hoping for some juice … oh well." He shut the door and leaned on it for a moment before looking blearily at the kettle.

"You're not fine. You're clearly ill … or poisoned."

John's eyebrows contracted. "Poisoned?"

"I don't want to alarm you, but I think you might have been exposed to something at Baskerville. I don't know what it is yet, but I've texted Mycroft. He was not helpful at all which is very unlike him, of course, but…."

Sherlock's voice broke off in alarm as John sat down and put his head in his arms, gasping and coughing weakly. "John! Quickly! Tell me your symptoms. The sooner I can isolate the poison, the better. There's clearly no time to waste."

John's breath wheezed even more, and his shoulders shook. Muscle spasms? Again, Sherlock felt fear.

And then he realized that John was laughing.

At him.

The laughter dissolved into a fit of coughing (and it served him right), but Sherlock was still indigant. "This isn't funny, John. You must be delirious."

"Oh, believe me, this is funny, Sherlock." John gasped out between coughs. "Hilarious, even."

Sherlock just stared at him. What could be so funny? His irritation climbed even higher as John started laughing again. "Did you say you textedMycroft about this? Oh my God. He'll never let you hear the end of this."

"It did seem the fastest way to get answers. I don't see why you think it's humorous."

"Oh, but it is, though I can't say I feel much like laughing at the moment." John bent in a fit of coughing and then sniffled mightily. "And you, of all people, so proud of OBSERVING things and not just seeing them."

"What are you talking about?" Concern was draining into irritation again.

"There's something you clearly didn't observe in Baskerville, Sherlock," said John as he gathered his feet under him. "I don't think I feel like making tea, after all. I'm going back to bed."

Speechless, Sherlock watched him go. He had missed something? HIM? Clearly John was feverish.

Which, he supposed, meant he was more likely to be sick than poisoned. John was a doctor, and he didn't seem overly concerned over his own health. If he were seriously ill, he would know … wouldn't he?

What had he said about Baskerville? Sherlock cast his fine memory back to their time in the lab. The daylight tour that was cut short by Mycroft's stolen pass. The nighttime experiment. Hmm… he had not been able to watch John his entire time while there. There had been time wasted arguing with the Major over access.

He pulled out his phone.

-Need footage of John's visit to Baskerville. SH

-Of course, dear brother. MH

The text came back within moments, and it was immediately followed by the video. Mycroft was being unusually helpful, not even a quibble about national security. Maybe he was concerned after all? Usually he had some kind of ulterior motive when he was this obliging.

Anxiously, Sherlock watched the video. There was John, casually touring the labs, using his pass to open locked doors. And … wait. Was that a sign on the door?

The doorbell rang, but he ignored it. What did that sign say? He could make out the "Keep Out" at the top, but it didn't look as if John had even noticed. Something about … He leaned toward the fuzzy screen, wishing for higher resolution, just as Mrs. Hudson came in the room.

"I keep telling you boys that I am not your housekeeper," she told him. "But you're not answering your bell. There was a delivery."

"Not now. I'm busy," he said, intent on his screen.

A resounding sneeze came from John's room and Mrs. Hudson looked up. "Poor dear. He sounds like he has a terrible cold."

Sherlock sniffed. As if something so pedestrian could make John so ill. Sometimes it amazed him that ordinary people could so much as deduce the weather by looking out the window, their minds missed so many crucial details.

Still squinting at the screen, he asked, "Delivery?" in a distracted tone.

"Yes, dear." She crossed over and put a shopping bag on the table. "It wasn't a normal delivery boy, mind you. He was dressed in such a nice suit. He said to tell you that your brother had sent it, with his compliments. It's not your birthday, is it?"

A package from Mycroft? Sherlock abandoned the phone and opened the carrier, expecting pages of notes, syringes of poison antidotes. But no. The bag was filled with what looked like ordinary cold remedies: capsules, cough medicine, vitamins. There were groceries, too, including orange juice, canned soup, and a box of tissues.

Rendered speechless, he just stared.

Just then, his phone dinged.

This time it was a photo. A crystal clear close-up of the door he had tried so hard to see in the video.

"KEEP OUT unless you want a cold."

Oh.

John was right.

Mycroft would never let him hear the end of this.