Gregor and the Curse of the Warmbloods by Suzanne Collins
The third in a gripping young fantasy series from the author of THE HUNGER GAMES. Gregor and Boots must return to the Underland to help ward off a plague. It is spreading fast, and when it claims one of Gregor's family, he begins to truly understand his role in the Prophecy of Blood. Gregor must summon all his power to end the biological warfare that threatens the fate of every warmblooded creature.