Lost World — Kate L. Mary

Lost World, the fourth book in the Broken World series, continues following a group of survivors after a super-virus has destroyed the world and the dead have come back to claim it as their own...

With their shelter destroyed and their sense of security shattered, Vivian, Axl and the group set off in search of a safe place. But the group's morale is at an all-time low, and they now know it's not just the dead they have to fear. Weeks of wandering brings sickness, death and loss to the survivors, and Vivian soon finds herself losing hope of ever discovering a safe place to live. To make matters worse, two members of the group go missing, leaving the others to wonder not only what happened to them, but if they will ever see their friends alive again. After what happened in Vegas, Hadley feels like a stranger in her own body. When she and Jon get separated from the rest of the group, she finds herself confronted not only with the scars left behind by her ordeal, but facing the possibility of a new beginning with Jon at her side. In a twist of fate, Vivian and Axl stumble upon a place that could be the group's salvation, only to have hope slip through their fingers once again when they are met with the most devastating blow yet…