The Enemy (An Enemy Novel)

A sickness has afflicted everyone over the age of 16; anyone who is a "grown-up" has become a decomposing, brainless creature that survives by feeding on children. The children and teens have barricaded themselves in fortified buildings, fighting off attacks from the grown-ups who travel in packs, like hungry dogs. Before long, the young survivors are promised a safe haven in Buckingham Palace and make their way to it, crossing London on a perilous journey that will test them in harrowing ways. But their fight to stay alive is far from over - the threat from within is as real as the one on the outside.