The Fallen (The Enemy, #5) by Charlie Higson

First the sickness rotted the adults' minds. Then their bodies. Now they stalk the streets, hunting human flesh. The Holloway crew are survivors. They've fought their way across London and made it to the Natural History Museum alive - just. But the fight will never end while the Enemy lives, unless there's another way. . . The kids at the museum are looking for a cure. All they need are medical supplies. To get them means a journey down unknown roads. Roads where not only crazed, hungry sickos hide in the shadows. SUDDENLY IT'S NOT SO CLEAR WHO - OR WHAT - THEY'RE FIGHTING.