The book does not name the illness that kills Ivan Ilyich. The text offers two clues that are never resolved: a dispute between doctors over a floating kidney and the appendix, and a fall from a small ladder while hanging curtains, which the sick man himself takes as the beginning of everything.
The fall is in chapter III. Ivan Ilyich climbs a small ladder to show the upholsterer how he wants the drapery, misses his footing, and catches himself; he only strikes his side against the handle of the window frame, the pain passes, and at night he laughs at the scare. The signs come in chapter IV: a strange taste in his mouth and discomfort on the left side of his belly. On the night of the discovery he retraces the history of the illness from the beginning, and the beginning is the tumble; in chapter VI, facing the curtain, he thinks that there he lost his life as in an assault.
The doctors resolve nothing. The famous doctor treats the case the way Ivan Ilyich used to treat the accused: for him there was no question of the patient's life, there was a dispute between the floating kidney and the appendix, which he decides in favor of the appendix. A friendly doctor later speaks of a little something in the appendix that can be corrected, and Ivan Ilyich returns cheered. The same night he understands that the case is neither the appendix nor the kidney; it is life and death. And in chapter VIII the famous doctor again speaks of kidney and appendix and, when asked, says only that there is a possibility.
So the two clues serve the reading, not the diagnosis. If the fall is the real cause, death is born from the most characteristic gesture of the life it ends, the zeal for the decorated house. If it is the cause Ivan Ilyich chooses, it is just one more explanation of an organ or an accident, of the same kind as the kidney and the appendix, and the text never says what he has. What the novella takes seriously is the lie around him: the chief torment of Ivan Ilyich is the lie, accepted by all, that he is only ill and not dying.