We are given this piece of information when the narrator finally arrives to the gloomy mansion that he will be sojourning for a while with his friend, Roderick Usher. Roderick tells the narrator, his friend, about the nature of his "malady":
It was, he said, a constitutional and a family evil, and one for which he despaired to find a remedy - a mere nervous affection, he immediately added, which would undoubtedly soon pass off. It displayed itself in a host of unnatural sensations... He suffered much from a morbid acuteness of the senses; the most insipid food was alone endurable; he could wear only garments of certain texture; the odours of all flowers were oppressive; his eyes were tortured by even a faint light; and there were but peculiar sounds, and these from stringed instruments, which did not inspire him with horror.
Note how this "malady" manages to impact all of his senses in a negative fashion, making him a virtual recluse and one who must live in darkness and away from living things. Interestingly, some critics have argued that both Roderick and Madeline are described in terms that make them out to be vampires or some form of undead creatures, but you will have to make your own mind up about that one!