Death Toll

Death Toll is a campaign set in a small rural area. In it, the Survivors travel through the countryside, a rural village, and have a final fight at some docks awaiting a boat to rescue them. Valve has talked the least about this campaign.

Official description
Shoot, shove and sprint through a nightmare suburbia consumed by the Infected horde. The Death Toll campaign has your team of survivors making their way down a turnpike littered with abandoned cars to the nearby ghost town of Riverside—the site of a failed stand-off between the last of humanity and a limitless swarm of undead. Fight through the ruins of small town America to the waterfront nearby, where a rescue party can take you upriver to safety.

Writings on the wall
Several safehouses throughout the map have some writings of time of death and plans to move to other places such as mercy hospital and the airport. We also see on the wall Exodus 9:15 which means "For if by now I had put forth My hand and struck you and your people with pestilence, you would then have been cut off from the earth" in the bible