The Enemy series by Charlie Higson

1. The Enemy:

In the wake of a devastating disease, everyone sixteen and older is either dead or a decomposing, brainless creature with a ravenous appetite for flesh. Teens have barricaded themselves in buildings throughout London and venture outside only when they need to scavenge for food. The group of kids living a Waitrose supermarket is beginning to run out of options. When a mysterious traveler arrives and offers them safe haven at Buckingham Palace, they begin a harrowing journey across London. But their fight is far from over--the threat from within the palace is as real as the one outside it.Full of unexpected twists and quick-thinking heroes, The Enemy is a fast-paced, white-knuckle tale of survival in the face of unimaginable horror.

2. The Dead:

A terrible disease is striking everyone over the age of fourteen. Death walks the streets. Nowhere is safe. Maxie, Blue and the rest of the Holloway crew aren't the only kids trying to escape the ferocious adults who prey on them. Jack and Ed are best friends, but their battle to stay alive tests their friendship to the limit as they go on the run with a mismatched group of other kids - nerds, fighters, misfits. And one adult. Greg, a butcher, who claims he's immune to the disease. They must work together if they want to make it in this terrifying new world. But as a fresh disaster threatens to overwhelm London, they realize they won't all survive.

3. The Fear:

The sickness struck everyone sixteen and over. Mothers and fathers, older brothers, sisters, and best friends. No one escaped its touch. And now children across London are being hunted by ferocious grown-ups who are hungry, bloodthirsty, and not giving up.

DogNut and the rest of his crew, in search of the friends they lost during the fire, set off on a deadly mission from the Tower of London to Buckingham Palace and beyond, as the sickos lie in wait. But who are their friends and who is the enemy in this changed world?

4. The Sacrifice:

The sickness destroyed everyone over the age of fourteen. All across London diseased adults are waiting, hungry predators with rotten flesh and ravaged minds.
Small Sam and his unlikely ally, The Kid, have survived. They’re safe with Ed and his friends at the Tower of London, but Sam is desperate to find his sister.
Their search for Ella means Sam and The Kid must cross the forbidden zone. And what awaits them there is more terrifying than any of the horror they’ve suffered so far

5. The Fallen:

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.