Queens of Tristaine by Cate Culpepper

When a deadly plague stalks the Amazons of Tristaine, Jess and Brenna must return to the place of their nightmares, the notorious City Clinic, to find a cure.

Tristaine's hard-won peace is shattered when a lethal epidemic threatens to devastate the Amazon clan. Their only hope for a cure lies in the sinister laboratories of the City Clinic. To save the sisters they love, Jess and Brenna must leave their mountain village and return to the malevolent government facility that almost cost them their lives. The danger they face is harrowing, the stakes enormous...and they have precious little time.

Book Four in the Tristaine Series