Fouldrought is a terrible disease that can infect creatures of Mistmantle and Whitewings. It first appears in The Heir of Mistmantle.. The cause of it is generally a poisoned water source. Otters seem to be immune to fouldrought. In The Heir of Mistmantle, the cause of it was a diseased fish that was dropped by a bird into a river that many creatures drank from.

Symptoms include severe headaches, high fever, vomiting, aching, swollen limbs, convulsions, and in some cases, a yellow or red rash on the creature's paws.

Many animals died from this disease during the outbreak in The Heir of Mistmantle, Including Juniper's mother, Damson.  Many caught the disease, but were cured, including Brother Fir, Yarrow, and Hobb.