Poxfulcrum

There is a legend that Nurgle keeps a she-daemon (some say his daughter), Poxfulcrum, caged in his workshop, and he tests the contents of his cauldron upon her, forcing her to drink the noxious sludge.

She is cursed with a vulnerability to every disease, but her body can also purge every infection afflicting it – the ease with which she recovers from Nurgle’s poisonous broth allows him to gauge its potency. Only when the symptoms of his new concoction please him does he unleash it upon the mortal world.

In her suffering, Poxfulcrum weeps, and it is said that each tear can heal every ailment in the world. When Nurgle is absorbed in his work, she is said to whisper to mortals the secret cures for each of his diseases in revenge for her confinement.

Source

 * Warhammer Fantasy RPG 3rd ED: The Book of Plague -- pg. 6