The rains

"In one terrifying night, the peaceful community of Creek's Cause turns into a war zone. No one under the age of eighteen is safe. Chance Rain and his older brother, Patrick, have already fended off multiple attacks from infected adults by the time they arrive at the school where other young survivors are hiding. Most of the kids they know have been dragged away by once-trusted adults who are now ferocious, inhuman beings. The parasite that transformed them takes hold after people turn eighteen--and Patrick's birthday is only a few days away"--Amazon.com.