Captives : book one of the Safe Lands series
(Book)

In a dystopian future, most of the population is infected with a plague. The exceptions are those who live outside the Safe Land's city walls. When a mutation in the plague sends city enforcers looking for the uninfected, Levi's village is raided. While the attack leaves many dead, Levi's fiancee is being held captive. Levi launches a war against the city in an attempt to free her before it's too late.