The Enemy is a book written by Charlie Higson. It takes place in London, England from the halfway point in the storyline; it is about a virus/disease that people over the ages of 14 are susceptible. 

When the virus attacks the victim turns into a zombie like creature. The uninfected children try to survive by fighting for their lives. 

The initial setting of the story begins with the children taking refuge inside of a grocery store.  One evening turned bad when the children in the grocery store receive an unexpected visitor who is telling them that there is a safe haven at Buckingham Palace. The group then runs into some unexpected bumps and turns on the road to Buckingham Palace.