From the Classics: Serpent of Changes

Two natures thus transmuted did he sing,
Wherein both shapes were ready to assume
The other's substance. They in mutual guise
So answer'd, that the serpent split his train
Divided to a fork, and the pierc'd spirit
Drew close his steps together, legs and thighs
Compacted, that no sign of juncture soon
Was visible: the tail disparted took
The figure which the spirit lost, its skin
Soft'ning, his indurated to a rind.

...

The soul, transform'd into the brute, glides off,
Hissing along the vale, and after him
The other talking sputters; but soon turn'd
His new-grown shoulders on him, and in few
Thus to another spake: "Along this path
Crawling, as I have done, speed Buoso now!"

- Dante Aligheri, Inferno 25

 

Dante describes the damned being punished by violent snakes, who have the power to transform their victims' bodies. When the soul is bitten by the snake, he begins to turn into a snake, while the snake turns back into a man (I have omitted most of transformation sequence, which is very long and uncannily reminiscent of Animorphs).

The Serpent of Changes is a monster known for its strange poison, which causes the victim to themselves transform into such a Serpent. The only way to escape this fate is to bite someone else, passing on the affliction. Only humans can be affected by the poison, and each person only once before they become immune. For each day that the victim remains in serpent form, they lose one point of Wisdom, until when their Wisdom reaches zero they have become one with the snake and can never turn back, though they will still attempt to bite and transform others.