Who is the antagonist in the novel, Zone One?

Although no skel personifies a single character, they are ever-present in the novel’s landscape, patrolling the peripheries of the present, always hovering, ready to bite. Skels are the zombie-like creatures that the plague created. Though they are considered to be dead by the survivors, they are alive, bloodthirsty and thoughtless, the shells of their former human forms. Much of earth’s human population has become a skel, and they are the target of the sweeper teams cleanup mission.