[insert disclaimer here: The closest thing I own to anything in this story is a Sound village headband. And even that's just a cheap Chinese knockoff.]
"Orochimaru, you promised…." Kabuto cursed as a thin white snake slithered over his foot. The omen was clear – what's done is done. Orochimaru had fled from the Akatsuki, and was en route to their hidden base. The omen, however, was also two-fold. Somewhere along the way, Orochimaru had performed the living corpse reanimation jutsu.
Kabuto shuddered. The thought of that alone filled him with dread.
He promised, Kabuto thought to himself, Orochimaru promised to execute the new jutsu in the safety of the lab, and under his close medical supervision. The technique was powerful, but it was so inherently dangerous that it made most forbidden jutsu look like child's play.
It required an immense, if not almost life-threatening, amount of chakra, and a slight mistake at any point in the execution of the technique could prove fatal before a bond is even formed. That, unfortunately, was only the beginning of the danger.
Unless the new body was carefully selected and specially treated in preparation for the transfer, Orochimaru's presence could be immediately rejected through a violent biochemical cascade. And even if it the preparations were absolutely perfect, there was still no possible way to account for the histocompatability of every system. Rejection wasn't a question of if. It was a question of when, and that question could only be answered by a strict regimen of immunosuppressive therapy. The correct balance was essential. Fail to suppress the immune system enough, and rejection could lead to mutli-organ failure. Suppress it too much, and Orochimaru could succumb to any number of opportunistic infections.
Essentially, from the moment the jutsu is first used, Kabuto would need to keep Orochimaru alive until he had recovered enough chakra to transfer into yet another body, starting the never-ending process all over again. Clearly, this was not a jutsu to take lightly.
Orochimaru knew all of this, and yet…
He must have been forced, Kabuto reasoned. The mission was to capture the elder Uchiha brother, alive, and return to the base to begin the necessary preparations. For Orochimaru to alter that mission, he must have been backed into a corner. He must have run out of alternatives. And if that was the case…
Kabuto's heart pounded in his chest as he furiously packed his field kit. He had to find his master immediately. There was no time to lose. If hyperacute rejection of the host body set in, he'd have three, maybe four minutes at most, before the damage done is permanent. Any longer, and Orochimaru would be dead.
Kabuto glanced down at the snake, and the snake understood. It shot past him as he leapt up the stairs, six at a time. The creature darted into the forest, and Kabuto followed.
Orochimaru must be close, Kabuto concluded. Investing enough of his remaining chakra on a summoning technique required at least some degree of confidence that he could make the rest of the trip without a fight. On the other hand, it might have only been a last resort. He may have sent the snake because he didn't have enough power left to make it back to the base on his own…
Kabuto was right, unfortunately… on all counts.
Out of time and almost out of chakra, Orochimaru's new body betrayed him in the worst of ways. Every organ system revolted against its new owner in a bloodthirsty war that no reasonable medic could hope to placate.
Kabuto tore open his kit as he slid to his knees. Orochimaru finally slumped, breathless and shaking, into his medic's arms. He wrapped one hand around his stomach as he spat another mouthful of bile into the dirt. He clutched his chest with the other as his heart strained under a frantic rhythm. His skin burned. His limbs felt heavy. His head felt like it was going to explode.
"Incompatibility with the host antibodies activated a complete systemic inflammatory response… thrombosis, loss of vascular integrity… tissue infarction is unavoidable." Kabuto rambled rapidly. He immediately injected an anticoagulant while reconstituting a 10 mL bolus of his most powerful thrombylitic agent. "Lord Orochimaru, hang on. I need to start an intravenous infusion before we can travel."
"Will… will that… make it stop..."
"Even a combined regimen of immunosuppressants and corticosteroids won't be enough to counter this reaction." Kabuto focused his healing chakra on lowering Orochimaru's body temperature and stabilizing what he could. "But I have enough of your bone marrow in the lab to do a full transplant. If I can force the host body to accept your immune system, we may be able to…"
"Just do it!"
Kabuto was practically beside himself.
"My lord, if the initial ischemic effects of multi-organ rejection don't kill you, then the continued inflammatory response of reperfusion injury easily could. Do you even realize just how dangerous a situation you've put yourself in?"
"Of course I do." he hissed, before finally succumbing to Kabuto's induced hypothermia. "But I… I also know… just how good my medic is."