Dragons are huge, powerful creatures feared by one and all, but just as the mightiest human warrior can be laid low by the flu, so too can dragons contract, be weakened by, and even die from various diseases. What follows is a description of some, but by no means all, of the maladies and draco-disorders afflicting the great reptilian race…
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Scale Lice: Scale lice feed under the scales of any scaled creature, such as dragons, hydras, giant reptiles, naga, mermen, and so on. Since the lice burrow under a dragon’s scales, it cannot use its breath weapon to remove them: the dragon scales themselves protect the dragon’s tiny tormentors. Scale lice cause large patches of discoloration, which annoys vain and egotistical dragons, and like the lice that affect humans, they cause a great deal of itching. Thus, the dragon is unable to sleep properly, and even normally friendly dragons can become extremely grumpy and out of sorts while unfriendly dragons may rage through the surrounding countryside venting their anger at such discomfort…
If a dragon should become lice ridden after adventurers arrive, it will blame them for the infestation and demand they cure it. Very little goodwill from the dragon can be expected in such a circumstance. However, underhanded adventurers might deliberately (but with extreme secrecy) infect a dragon with lice in order to show up later with a cure and thus gain the dragon’s favor. It is told that one bold band of heroes left a small crate of rare fruit beloved of dragons on the trail. The crate appeared to have fallen unnoticed from some wagon. While the dragon took the greatest of care in examining the fruit for any abnormalities, the crate itself was booby trapped with scale lice. Weeks later, when the adventurers found the miserable creature, he was more than ready to come to any terms for a cure that might rid him of his malady. Should a dragon become aware that it has been duped and deliberately infested, woe be to any of the deceivers who fall into its clutches.
Dragon hide with scale lice discoloration sold to armorers brings only about 1/2 the price of normal or unblemished dragon hide. The scale lice have no effect on the overall effectiveness of the hide used in armor, but armorers perpetuate the myth that it does in order to reduce the price they pay.
How do you cure it? An industrial tub of mayonnaise?
I feel a module coming on. First Scene: a dragon lands near the town, rolling over and over, itching to death. It knocks down a few towers and deliriously thrashes about, ruining the mayor’s “deck with a view” on the top of the city wall. It screams, “There’s millions of them!” Then it takes off again only to crash on Farmer Domi’s barn, smashing it and half his livestock. This continues as he blunders out of sight.
The repel vermin spell would remove them. That’s the most obvious way to cure it.
Long-term heal checks at a certain DC could also do it, though I doubt if the dragon would be willing to submit to it.