From the symptoms he has shown so far, my guess would have to be a type of blood illness. maybe around the lines of cancer. It seems to hit him more often then normal and it doesn't look like it make him throw up. it just takes his energy. Either that, or it's probably just that he has a very bad immune system, like a low white blood cell count or something around those lines.
It's not easy being Hollow with a fully removed mask.
Ukitake's sickness is something that has always bothered me. Why can't it be cured or him be healed? With brains like Urahara or Mayuri or others, why he is in this condition? I have often wondered if it is meant to have more significance than we imagined.
I thought I read somewhere that, as a child, Ukitake suffered from tuberculosis and was never able to fully recover from it.
Ukitake has always been like that, I guess he has some sort of genetic defect which can't be fixed even by Inoue. His illness played an important role in Rukia's flashback with Kaien, I doubt it'll have more significance in the story other than the fact that Rukia will have to assume more responsibility in the division, which may prove critical if Ukitake gets sick at the wrong time.
He has been sick since he was a child if I remember right. It's called tuberculosis and he just never recovered from itor something like that
To the best of my knowledge, it has never been mentioned that it was tuberculosis but since he was coughing out blood, many people thought it was the most likely reason. We know that he was sick even when he was in shinigami academy, it's safe to assume that it's an inborn disease, tuberculosis, on the other hand, is an infectious disease. I don't think Ukitake's illness resembles anything that is known to man.
Ukitake is too haxed with the ladies so somebody decided to make him sick so that the other guys have a chance
I think the disease might have a significant meaning. If anyone read Deeper Samurai kyo, you would understand. The sickness.could be due to a strange reactive reiatsu or that ukitake is not a complete shinigami in a sense. Honestly after reading.bits of the bleach novel about Kubo putting in hints, i would.not.be surprised