The Hunted — Charlie Higson

The sickness struck everyone sixteen and older. First it twisted their minds; next it ravaged their bodies. Now the sickos roam the streets, crazed and hungry for young flesh.

Ella's friends had told her that the country would be safer than the city. They were wrong. Now they're dead and Ella's all alone--alone, that is, except for her silent rescuer, Scarface. But she doesn't even know if he's a kid or a grown-up.

Back in London, Ed is determined to find Ella and keep his promise to Small Sam that he will reunite sister and brother. But getting out of town has never been more dangerous. It seems that every sicko in the country is coming from all directions, almost as if they're being summoned to the capital. Will anything be able to stop the invading horde?