Gallomarg, Disease-ridden Serpent of the Maw

Covered in sores and pox-riddled pustules that crack and ooze, Gallomarg is cursed by a great many diseases, none of which seem to affect it, but instead plague those around it. It lives in a state of perpetual agony yet never dies from its own canvas of pain. If it could, it would die, but the great beast continues to lumber on in continual misery, restored by the wasting disease that exponentially replicates within its host.

Gallomarg's unique disease, something the locals have started to call "walking pestilence," works in concord with the dragon while most likely controlling a small part of its instincts. The virus cannot survive if the host dies, and Gallomarg seems the only creature hardy enough to have lived for centuries with this accursed affliction. Other creatures, particularly humans, who are exposed to walking pestilence never gain the regenerative powers that Gallomarg possesses and instead wither into husks of their former selves before rotting away, their bodies a sea of sores and seeping wounds that follow their vein lines as if their blood is boiling.