World of Ash - Ash and Ruin Trilogy, #1

There are two inherent truths in the world: life as we know it is over, and monsters are real. The Pestas came in the night, spreading their pox, a deadly plague that decimated the population. Kat, one of the unlucky few who survived, is determined to get to her last living relative and find shelter from the pox that continues to devastate the world. When it mutates and becomes airborne, Kat is desperate to avoid people because staying alone might be her only chance to stay alive.
That is, until she meets Dylan. Dylan, with his easy smile and dark, curly hair, has nowhere to go and no one to live for. He convinces Kat there can be safety in numbers, that they can watch out for each other. So the unlikely couple set off together through the barren wasteland to find a new life - if they can survive the roaming Pestas, bands of wild, gun-toting children, and piles of burning, pox-ridden bodies.