Disclaimer: Just to be clear, this isn't about the story, but about Rukia's illness, in case it wasn't explained properly. I don't own Bleach, I don't own Byakuya Kuchiki, and I don't own anything to do with Tite Kubo. Just so that we're clear.
Please review :)
Rukia's illness: -
Thunderstorms affect Rukia's immune system; which is why she gets a cold when one is over.
The hollow she was slashed by in the first chapter had been infected with a parasite called the Soul Leech, and it was embedded into her bloodstream. This parasite takes blood from various areas of her body, and moves it into the lungs, which gradually builds up over time.
This is why she started to vomit blood, when her lungs started to overflow. The Soul Leech is undetectable, and works quickly, feeding off reiatsu and blood.
It is the size of a 10p, or a dime. Rukia's pulse stopped when it took blood straight from her heart, causing her heart to stop. However, she didn't die.
When blood from the heart was taken, her pulse slowed right down, her heart then beating once every 30 seconds.
The parasite uses the reiatsu and blood to make eggs, and when they hatch, the offspring eat their way out of the body. This causes the host no pain, only causing a tingling much like pins and needles, and the holes heal up instantaneously, not even leaving a scar.
This was developed by Kurotsuchi, as a way to harvest reiatsu, but he didn't tell anyone. Only one of its kind, and Kurotsuchi later removes it for experiments.
Thunderstorms in the Soul Society are active reiatsu producers.
One last thing. The blood taken from other areas to the lungs is because the parasite wants to drown the host, and then use the corpse as a living place.
Hope that's clear. Please review! Any more questions, just ask.