Frenzy / Robert Lettrick.

Enhanced descriptions from Syndetics:

14-year-old Heath Lambert is spending his summer at Camp Harmony in the picturesque Cascade Mountain Valley. It's the perfect place to enjoy the soothing calm of nature as he weighs a heavy decision. The camp offers distractions: his friends, Cricket and Dunbar, always up for trouble; his reluctant crush on Emily, one half of the beautiful Em & Em Twins; and hulking bullies Thumper and Floaties, who are determined to make him their punching bag for the summer. But no one rattles Heath like his creepy cabin mate, Will Stringer. Brilliant, cold and calculating, Will views the world as one big chess game, and he's always three moves ahead of everyone else.

Heath soon learns there's a much bigger threat to contend with. Something's wrong with the animals in the surrounding forest. A darkness is spreading, driving them mad with rage. Wolves, bears, mountain lions-even the chipmunks are infected, spurred on in droves by one horrific goal: hunt and kill every human they find.

Heath and a ragtag band of campers are faced with a choice: follow Will's lead and possibly survive, or follow the camp staff and die. But how do you trust a leader when you suspect he's more dangerous than the animals you're running from?

Heath came to Camp Harmony to be surrounded by nature. He's about to get his wish.