Only Shadowhunters can be affected with this disease; it has been assumed that it is caused by the interaction of demon poisons with the angelic nature of Shadowhunters.
The first symptom seen in an afflicted Shadowhunter is a shield-shaped rash on the back that spreads over the body, creating fissures in the skin. The affected will deteriorate physically and will experience fever, chills, nausea, oozing sores, non-oozing sores, buboes, a film of black over the eyes, hair ejection, skin discoloration, swelling, and other similar signs of distress. The demon pox would then aggravate and may lead to insanity. The sores and fissures will eventually cover the skin of the victim entirely, forming a dark chrysalis within which the victim transforms into a demon. After several weeks of supposedly painful transformation, the demon will emerge from the chrysalis.
In earlier times, the disease was lethal and had no cure, with treatment focused only on slowing the progress. Today, cures exist that can clear up the infection, but only if treated in its early stages.
Its presence is apparently sometimes considered evidence as violation of the Law against consorting with demons, thus those who are treated for demon pox often receive treatment while imprisoned in the Silent City.
- Benedict Lightwood — Benedict's condition was severe enough that he needed medication supplied to him by Axel Mortmain. This failed and he eventually turned into a gigantic worm.
- Barbara Pangborn — Barbara contracted the pox from her husband, Benedict; she committed suicide out of shame for contracting the disease from her husband.
In the 1870s, Will Herondale had an immense interest in demon pox and was passionate about proving its existence. When it was proven to be a real disease, Will was ecstatic and made up a song about it. He later taught the song to his children and grandchildren.
Just how is it acquired?
One must go down to the bad part 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!”
To prove that demon pox warps the brain.
So though 'tis pity, it's not in vain
That the pox-ridden worm was slain:
For to believe in me, you all must deign.”
- Demon pox is based on syphilis.
- Not all demons give demon pox; or at least, not every sexual contact with a demon will result in contracting the disease.