Brás Cubas died of pneumonia, but he himself attributes the death to an idea, that of the plaster. In the first chapter he says: "I died of pneumonia; but if I tell you that it was less the pneumonia than a grandiose and useful idea that was the cause of my death, the reader may not believe me, and yet it is true."
The idea is the anti-hypochondriac plaster, presented in chapter II. In chapter V, Brás explains the chain of events. Busy preparing the invention, "I received full in the face a blast of air; I fell ill at once, and did not treat myself". The next day he was worse, treated himself "incompletely, without method, care, or persistence", and from that came the malady that took him. He did not feel the pain in time because, he says, "I carried with me the fixed idea of madmen and the strong". He claims to have proved that the invention killed him and at once weakens the proof: "There are less lucid demonstrations, and none less triumphant."
He died on a Friday in August 1869, at two in the afternoon, at the Catumbi farm, at sixty-four and unmarried. Ill, he received a visit from Virgília and had the delirium that occupies chapter VII. In the last chapter he returns to the subject: the plaster "died with me, because of the illness I caught".
The book thus gives two causes side by side: the medical one, the pneumonia from the blast of air, and the moral one, the vanity of one who wanted to see his name on the remedy's boxes. Brás joins them with an irony that does not decide which of the two weighs more.