The Irresistible Urge to Fall for Your Enemy - Dearly Beloathed Series - Book 1

When Osric Mordaunt, member of the Fyren Order of assassins, falls ill, he realizes he needs the expertise of a very specific healer. As fate would have it, that healer belongs to an enemy faction, the Haelan Order. 

Aurienne Fairhrim and her fellow Haelan are inundated by sick children suffering from an outbreak of a long-forgotten Pox. Unable to get the funding needed to launch an immunization program, the Haelan Order is desperate for money - so desperate that when Osric breaks into their headquarters to bribe Aurienne to heal him, she is forced to accept. 

As Osric and Aurienne work together to solve not only his illness but the mysterious reoccurrence of the Pox, they find themselves ardently denying their attraction, which only fuels the tension between them.