From the Infernal Devices series by Cassandra Clare, it's a disease mentioned repeatedly by Will, one of the main characters. At first, everyone thinks he's joking, but in the second book... well, it's a great moment.
Demon pox is a very nasty disease.

Demon pox, oh demon pox,

just how is it acquired?
One must go down to the bad side of town
until one is very tired.

Demon pox, oh demon pox,
I had it all along,
no not the pox, you foolish blocks,
I mean this very song -
for I was right, and you were wrong!
by mackenziebarrett January 3, 2012
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A STD you may contract after having intercourse with a demon. You will turn into a demon and die if this STD is contracted
Will: hey I heard Benedict Lightworm got Demon Pox
by William Herondale November 8, 2019
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