The End (The Enemy #7) by Charlie Higson

It all comes to an end in the final book in The Enemy series The sickness struck everyone over fourteen. First it twisted their minds. Next it ravaged their bodies. Now they roam the streets - Crazed and hungry. Sickos swarm the streets of London. Gathered in the centre of the city, they lie in wait. The survivors have one final epic battle to overcome. Together they must work out a plan of attack and end the grown-ups reign of terror before it's too late. The end is coming.