Why are Usher’s and his sister’s maladies never identified

The story borders on the metaphysical so their maladies are not strictly identified. There must be some otherworldly mystery about them. In "The Fall of the House of Usher," it is not clear to what extent the supernatural can be said to account for the strangeness of the events in the tale. Madeline may actually have died and risen like a vampire--much as Usher seems to possess vampiric qualities, arising "from a sofa on which he had been lying at full length" when the Narrator first sees him, avoiding all daylight and most food, and roaming through his crypt-like abode. But a more realistic version of events suggests that she may have been mistaken for dead--and luckily managed to escape her tomb. Either way, the line between life and death is a fine one in Poe's fiction, and Usher's study of the "sentience of all vegetable things" fits aptly with Poe's own preoccupations.