Original Prompt: Post "City on the Edge of Forever", McCoy starts coming down with a disease (scarlet fever, diphtheria, TB, even smallpox) which has been extinct for so long that no one aboard ship has the antibodies for it anymore. Kirk and Spock must help him create a vaccine before the entire crew finishes the incubation period, even though that means keeping McCoy on his feet and working even if it kills him.

Incubation

By Swiss

Incubation Phase - 14 days from exposure

On the stardate that everything changed, Spock was eating with McCoy in the refectory. The doctor had assembled a breakfast tray from the replicator and now sat rubbing the back of his neck in a way that suggested stiffness. One elegant eyebrow raised, Spock inquired, "Are you feeling unwell, Doctor?"

The scowl McCoy favored him with lacked its usual force. Wearily, he pressed the ridge between his eyes, saying, "I'm fine, Spock. Just having a hard time waking up, that's all."

If anything, Spock came to even greater attention. It was unusual for McCoy to answer even his most well intentioned inquiries with such directness. "Did you sleep poorly?"

It was what the captain would have called a 'loaded' question. He knew, in fact, that neither Jim nor McCoy had been resting properly in the two weeks since they returned to their proper time through the Guardian of Forever. Edith Keeler's death still haunted Jim, and he rarely spoke to either of his old friends. Spock, because his logic had proved too brutal, and McCoy... Perhaps it was because McCoy had initiated it all, however unintentionally.

They had restored the future, but that fact was inconsequential to their captain's very human grief. A human himself, McCoy seemed to both understand and accept this, and Spock endeavored to follow his lead. He provided "space". As a result, he had been spending more time in the doctor's company. Vulcans were no more solitary beings than humans, after all.

"Look around, Spock," McCoy said, burying his face in his coffee mug. "I'm not the only one dragging."

A number of the crew were sagging in their chairs, inhaling caffeine supplements between yawns. "They are exhibiting characteristics of lethargy, a condition that is likely to pass. Your own weariness is more profound." He paused, just momentarily, before adding, "And you do not look well."

The doctor exhaled harshly, scrubbing his face with his hands. Spock noticed an odd flush peeking out of his collar, streaking into his hairline. "Yeah, well, I might be coming down with something."

Spock suspected this was the truth. McCoy had made an unprecedented recovery from the Cordrazine overdose, but the strain on his body could easily have made him vulnerable to an opportunistic infection. Perhaps even –

An unknown chemical reaction flushed Spock's glands, creating a sense of foreboding. The hairs on his skin prickled, and his heart beat a little harder. Perplexed by his own reaction, he reassessed the doctor's condition. There was a notable ashen cast to his face. He was also restless, engaging in the barest movement of his shoulders, raising and lowering them, stretching his back, then slumping down. Now that Spock was looking, he also noticed that, in spite of his lack of color, high on his cheeks were two bright spots. Fever.

"Perhaps you should run a diagnostic when you begin your shift in sickbay."

He expected backlash, some misplaced vitriol about Spock presuming not to know who the doctor was between them. None came. Instead, McCoy closed his eyes, which were lined with creases. "Yeah. Yeah, you're right, Spock. We'll see."

In that instant of easy acquiescence, Spock began to feel something very like genuine concern.

He became outright alarmed when, upon attempting to stand, the doctor's feet became tangled in the legs of his chair, causing him to stumble. He caught himself – muttering unintelligibly about gracefulness and China shops – but there was a dazed look in his eyes when he straightened, and his movements remained stiff and awkward.

"Doctor?" Spock asked, already beginning to stand.

McCoy brushed him off irritably. "I'm fine, Spock," he insisted. "Just – just tired."

Witnessing how his pallor had become extreme, Spock said, "I shall escort you to sickbay."

"You'll do no such thing. You're due on the bridge in four minutes, sixty-five seconds."

"Seven minutes and twelve seconds, actually, Doctor."

A huff, like a laugh filtered through exasperation. But very weak. "Still, duty calls."

"I have another duty to my crewmates. Currently, my responsibility for your wellbeing is a greater priority."

"Sweet talker," the doctor murmured, but by then he had become truly disoriented, bowed over his hands and the table, his breathing labored. "But really, I'm -"

And that was when the blue of the doctor's eyes twisted up into his head and his knees slipped out from under him. Only Spock's proximity saved him from banging his head, and when the man was in his grasp, Spock felt that he was hot, his temperature far greater than was healthy for a human. Spock shifted the doctor's weight higher against his shoulder and called for medical attention.

James Tiberius Kirk was stretched out on his bunk, disrobed except for his regulation trousers. In spite of having his head pillowed on his arms, he wasn't sleeping. Instead, he gazed intently at the ceiling, staring down his thoughts, which somehow always managed to spiral back to the ache which had settled in his chest.

Edith. Her name brought up memories of brown curls and passion for things beyond her time. He longed for her in a way he couldn't explain. And how could he? Who could possibly understand the feelings of a man who had met his love in the past and then sacrificed her to the whim of history? It could not be articulated in any official report.

If circumstances were different, he might have spoken to the man who had long been his counselor in times of grief or stress, but right now, even seeing Bones…

From the desk, his computer console buzzed. When he didn't respond, the message relayed anyway. "Captain? You have a page from sickbay."

Kirk turned over in his bed and put his back to the comm.

"He's not responding, Mr. Spock. Shall I send a Yeoman?"

"Not at this time, Lieutenant," Spock answered, frowning even as he released the call button.

McCoy had not been long unconscious, and Spock had stayed beside the gurney as they traveled to sickbay. The man awoke just as they were entering through the doors. Spock expected indignant bluster, recriminations, demands to be set down. Instead, McCoy had remained eerily fever-clouded as he reached to grip the sleeve of Spock's tunic. "Spock?"

Even more telling, McCoy had not attempted to minimize the seriousness of his collapse, even after emergency rehydration had diminished the worst symptoms. He'd submitted to a battery of tests, a few of them ordered of his volition, and then they'd waited. Dr. McCoy was just reviewing the results when Spock returned from his unsatisfying attempt to contact the captain. He crossed to where the doctor sat. He appeared stunned.

"Spock, I need you to verify these results for me."

Spock required a moment for the uneasy feeling to flow over and through him. Within the realm of medicine, McCoy only very rarely requested that Spock review his findings. They must be very serious if he was hoping that Spock's own probing might contradict his conclusions.

Exactly one hour and forty-three minutes later, Spock made another call to the captain. This time, when it was ignored, he submitted his cabin override to the computer and had Uhura send someone in person.

Kirk wasn't happy about being summoned from his cabin, and he was still dealing with that undercurrent of annoyance when he made it to sickbay. To his surprise, he was stopped outside the doors by a frazzled technician who informed him that sickbay was under special containment. To enter, he would have to change into a protective suit. Kirk kept calm out of habit, unwilling to jump to conclusions, but his heart was pounding harder than usual when he finally made it to Bone's office.

He was taken off guard to find both McCoy and Spock waiting. They looked up when he entered, and Kirk found himself stumbling over his attempt to avoid McCoy's eyes. Pity or pain; whatever emotion was lingering there, he didn't want to see it.

"Gentlemen," Kirk said. Even as he spoke, he knew his greeting was slightly off; a little too stiff, a little too formal. Nor did he take a seat on the edge of McCoy's desk like he ordinarily would have. Actually, of the three of them, only McCoy was seated. He was slouched against the back of his chair, and now that Kirk was looking, the lines around his eyes seemed more strident than usual. And Spock was flanking him, standing just over one shoulder. It was an obvious position of support.

"What's going on?" He gestured to the jumpsuit he was wearing, which sat uncomfortably on his shoulders. He noted uneasily that McCoy was not wearing one.

"Captain," Spock began speaking. "We have discovered something which may have serious repercussions for the Enterprise. This morning, Doctor McCoy experienced a syncopal episode in the refectory."

"He fainted?"

Kirk looked at his Chief Medical Officer again, and suddenly realized that it was more than just stain that he had seen before. He didn't look well; there was a diminution, as though he had recently lost weight. The difference was so stark that Kirk wondered how long it had been since he'd really seen Bones.

Unwilling to address that can of worms, he retreated to a place a safety, behind the captain's stripes: "I expect you have a point, Mr. Spock."

He sought explanation in Spock's even features and found his conclusions disconcerting. For all that his friend seemed to show no expression, Kirk saw the folded arms and the slight downward turn of his mouth and knew that he was disturbed. Something was very wrong.

"I'm sick, Jim," McCoy put an end to his speculation. He had two fingers pressed against the corner of one eye, and his mouth was a grim, straight line.

Kirk felt a flutter of anxiety, yet he was still able to speak evenly. "I assume you mean something worse than a chest cold."

McCoy seemed hindered by a dreadful reluctance, and Kirk suddenly wished he could get up and walk out of the office before he heard what was coming. Unfortunately, escape wasn't an option. He was the captain, and if something had happened that was bad enough for Bones to seal the doors of sickbay, he needed to know.

Then Bones opened his mouth, and the hammer came down. "It's smallpox."

For a moment, Kirk was certain he'd misheard. "Smallpox," he repeated. "Is this a joke, Bones?"

"It was a disease of old earth." Spock supplied greater detail. "Throughout human history it was responsible for outbreaks that killed millions of beings. It has relatives on other worlds – Argelian Pox, Rigelian Blight – but in Terran form, there have been no known cases for more than three hundred years."

Kirk looked inside himself for a reaction but did not find one. Dismay and denial should have been there, even panic. But, no. He didn't feel anything yet. "If it was wiped out three hundred years ago, then how is it possible for it to be on my ship?"

There was a pause in which Bones looked as though he wanted very much not to answer. Finally, he dropped his head. "In 1930, there were vaccinations for smallpox, but not everyone got them."

It was like being hit by atmospheric backlash; the reminder was so sudden that Kirk momentarily lost his breath. 1930. The Guardian and all that had taken place in that not-so-inaccessible past. "Bones," he said. "You're saying that during that time, when we were –"

Bones looked at him with open sorrow. It was written all over his face. "Smallpox thrived where people lived packed together, especially in populations with poor health and hygiene. Spock's unique biochemistry should protect him, and I'm sure you maintained better cleanliness and diet than most in that time, Jim, but –"

"The Cordrazine overdose caused hysteria, delirium. Doctor McCoy does not remember everything that occurred while he was under its influence. We can only speculate about the conditions though which he may have wandered while incoherent of his surroundings. It would have left him very vulnerable."

White noise was filling Kirk's ears. His mind fought through it, thinking rapidly. "You said there was a vaccination. If it's in the database, then you should have access to it, right?"

Once again, it was Spock who answered. "The disease was rendered effectively extinct, Captain. At one time it was considered a threat for biological terrorism, and there were laboratory samples, but knowledge of this type was sometimes lost during the Eugenics War. It is my hypothesis that, since it was deemed obsolete, that data was never recovered. It is possible that no such vaccine exists."

The weariness McCoy was projecting had new meaning. Kirk looked at him, expecting to feel fervent worry, but there was something blocking the emotion. Even after Spock's pronouncement, he was still struggling to grasp the ramifications. "How lethal is this thing?"

"The disease was always dangerous," Bones said. "Before it was eradicated, it killed more than all our World Wars combined. And it'll be even worse now. No one on this ship will have encountered the disease. Their grandparents' grandparents didn't know smallpox."

The words had their own potency, but it was the bleakness of McCoy's pallor, the scarlet flush peeking out from his uniform, that was finally able to make Kirk afraid. "Bones," he asked. "What exactly are you saying?"

"I'm saying that, even in its time, there wasn't a treatment; people survived or they didn't. But now, when we're so completely unprepared –" He looked his captain in the eye. "You can expect the fatality rate to be unusually high."

"30 percent was the usual statistic, though fatalities in children were closer to 80 percent. It is more likely that we'll see fatalities reach the latter number," Spock said.

"And –" Once again Bones faltered, his entire body a vessel of guilt. "It transfers communicatively. There's a distinct chance that there is already an incubating population on the Enterprise."

"How long until we know for certain?" Kirk asked.

"A few days. After that, people will start showing symptoms. I've already had quarantine procedures implemented in sickbay, and Christine has started calling back crewmen who might have had contact with the disease. Anyone who spent time in an enclosed space with me will need to be ruled out for contamination."

The very idea made Jim go cold. "How many people are we talking about?"

"Jim," McCoy sounded faint. His expressive hands gestured helplessly. "I'm a doctor. I've had my hands on dozens of patients in the last week. I've had my hands on you."

Suddenly Kirk was angry, no longer unable to feel confounded by this unexpected threat to his ship, but truly angry in a way that transformed his handsome features. He saw the moment it registered to McCoy. He looked stricken.

"If you're sick, then it will get worse, right? You have to do something. How many days can we reasonably expect before you aren't able to function?"

Spock probably could have reported it to the day, the hour, yet he allowed McCoy to give his own prognosis. "Reasonably, Jim?" McCoy repeated. "To be showing symptoms, I'm already in the Prodromal phase. Perhaps five days before the actual pox breaks out."

"The doctor is being generous." Spock's contradiction was unusually gentle. "Though the data is incomplete and, in some cases, contradictory, it seems more likely to expect a timeframe of no more than three days."

"Two or three or five, it's all the same." Bones pressed his hands to his face. "I can barely keep my eyes open now. And my palate –" Already his voice sounded hoarse, as though his throat were swollen. "After the vesicles come up, I just don't know what will happen."

He must have been frightened, yet Kirk found himself unable or unwilling to reach out a hand or to bolster his spirit. The anger was still too strong. "What other resources do we have to figure this out? We have more than forty people in science."

"Geologists, Jim. Botanists, physicists, anthropologists. We've got the gambit. We can pull the people in Bio, and the chemists, pair them with medical, but –"

Bones dropped off, a tether that Spock took up. "There is only one person qualified in the multiple disciplines needed for the deconstruction of a disease and the hypothetical synthesis of a treatment or vaccine."

McCoy slumped further. "I've been hollering to Starfleet for an assistant CMO for months, but they've been putting me off. I tried to convince them it's not reasonable to have only one medical officer onboard with four hundred people to look after, but they don't want to approve the modified crew compliment –"

Kirk snapped to shut him up. "We have three days. Can you do it? Are you able?"

Bones flinched. But able? He would have to be. Resolutely, he pulled himself out of his seat, fumbling for the edge of his desk. He already looked uncomfortable. The worst thing, though, was that Kirk couldn't bring himself to look at McCoy except through a veil of surging adrenaline, of subdued rage. From the corner of his eye, he saw the disapproval in Spock's face.

"Three to five days," Kirk repeated. "I guess you'd better get busy, Doctor."