The Plague is a Underland sickness first introduced in Gregor and the Curse of the Warmbloods. The plague only affects warmblood creatures and so is named the curse of the warmbloods. A troup of characters, including Gregor, Boots, Temp, and Ripred must journey to a field in which the said cure of the plague grows. Once they are there the group is attacked by giant ants and several die, including Frill and Hamlet. The remaining survivers then conclude that Dr. Nevive, who is supposedly the main leader in developing a cure for the plague is actually the scientist in head of creating the sickness, along with Solovet, to use as a weapon against the Gnawers. While Ares, Gregors bond, was visiting the developing lab he accidentaly knocked over a jar containing plague infected fleas. This, the party concludes, is why he was one of the first to be infected. upon returning to Regalia, they alert the rest of the city to this newfound information and Nevive is killed and Solovet descrased.
Symptoms of the plague included large purple bumps on the skin (that would occasionally burst), a white tongue, and (in the bats, at least) loss of fur. Difficulty breathing may be one as well, however it may just be an effect from the victim who's suffering extreme pain, and is simply trying to keep themselves alive.