How We Became Wicked by Alexander Yates

Synopsis: After the world becomes infested with the singers, there are three kinds of people: the wicked, the true, and the vexed. The wicked have been infected by the wickedness spread by the singers and are out to kill. The true are people who haven't yet been infected. The vexed are a small group of people who are immune to the singers and the wickedness after getting bitten as a newborn.