The Kyuubi no Kitsune was confused. His millennia of experience did not prepare him for the sickness that suddenly gripped his vessel. No matter how much of his chakra he sends to his vessel's system to get rid or overpower the problem, it was continuously rebuffed as if an unknown force shielded his vessel's body from it, and no matter how hard he tried, which was exceptional in his case, nothing bore fruit.
Oh he was trying hard, oh yes he was. The Kyuubi felt fear for the first time in his entire existence for he knows when his vessel succumbs to death, so would he due to the Shinigami's god-forsaken seal that trapped him inside the boy.
Despite all the things he did to his vessels body after the sealing to ensure that he survives whatever ordeal he will face when he grows old enough to be on his own, this blasted fever was slowly killing his vessel and no amount of regeneration and increased chakra capacity can do anything about it.
So while his vessel suffered for hours under a very high fever, Kyuubi was doing his damn best to manipulate his energy towards his container just to get rid of the problem. Hours he worked but nothing seemed to work. Desperation forced the immortal being to work harder but all his efforts were futile.
In the end, Kyuubi sighed and pulled all his energy back into its body and waited for his death to come.
"What an idiotic way to die. Me, the Kyuubi no Kitsune, the most powerful Bijuu, death due to fever. I don't know if I should feel humiliated or disturbed." Kyuubi grumbled. He sighed again and gazed back at the pipes running through the ceiling of the sewer-like cavern that he was imprisoned in, noting absently the brownish energy appearing and disappearing along the pipes. He shrugged it off as nothing.
He was about to go into a nap when the same energy pulsed again.
He raised an eyebrow at the phenomenon. His curiosity peaked.
He focused the limited senses he had on the body of his vessel and noticed that instead of the body deteriorating due to sickness, the unknown energy was slowly improving his host's body.
"Interesting. It seems that my vessel isn't dying after all but what is happening to him?" He mused as he stretched his senses again to check on his vessels current condition and was surprised at what he found. All his years alive in the world did not prepare him on what the energy was doing to his host's biology.
Chakra coils were being improved especially towards the major senses.
The unknown energy was saturating his vessel's cells with purposes unknown, guessing that it was actually improving everything it touched or even changing to purpose unknown.
"Interesting indeed. It seems that my time in this world is not yet at an end. We shall see where this development will lead. Surprise me my vessel on the time that we meet." Chuckled Kyuubi as he laid his head down on his paw for a nap, no longer afraid that death would claim him during his rest.
Naruto woke up from his feverish sleep without any concept of time. He can feel his weakened body protest as he tiredly swiveled his head to check on the clock beside his bed so he can determine how long he has been asleep. His eyes widened in shock when he saw the date.