Acute intermittent porphyria
I didn't know there was a disease called "porphyria". The name piqued my interest because of Porphyria's Lover, an early example of Robert Browning's ability to arouse (the business with the soiled glove and the shoulder) and terrify (the speaker's cool madness and intimate tone). (Here it is as a sort of comic book.) The disease was named some 50 years after Browning wrote the above and (says a quick google) is not named after it. Rather, the (interesting - associated with the British royals but not vampires) set of syndromes seems to owe its name to a fancy word for purple (because some variants cause discolored urine) from the "Greek porphurā, shellfish yielding purple dye". I don't know why Browning chose the name, except perhaps because the word is beautiful and uncommon (the gloves may suggest she's an upper-class woman.) Browning had an eccentric and deep classical education - it often gets in his way, in my view (see also, P.B. Shelley). He's also too willing to mess with word order (as the above poem demonstrates at a few points) and general pronounceability, and The Ring and The Book is just tiresome after a few hundred pages. Still, a great poet, unlucky (like Tennyson) in his predecessors.
I note in passing the poem's last line: "And yet God has not said a word!"
Update: if you got here looking for an iconic image of Woody Guthrie with his "This machine kills fascists" guitar, click the link. Google has weirdly used the comment below as the source of the image here.