In Plague there are two ways to play the game. One is from a survivor point of view where you need to stay away from the zombies in the physical world while picking up randomly placed items that will help your survival. These items can range from cures to weapons.

The other way to play is as a zombie. Zombies are dumb in that they cannot pickup any items. But they can sense humans further away, the more humans they bite, the further away they can sense humans. They also get notified when a human enters the area (a sense that zombies have that humans don't). If a zombie bites a human, the human has 1 hour (real time) to find a cure, or find somebody who will administer a cure to them. Otherwise they will join the ranks of the undead. When you change into a zombie, you will also drop all of the items you have at your present location for a lucky survivor to grab.

Zombie CAN be incapacitated by humans from the various weapons. A incapacitated zombie cannot bite a human for 10 minutes. In that time you can either run away, or pump him/her full of cures which will turn them back into a human.

There is no end to the game but just a shift in perspectives, the longer to stay a human, or the longer you stay a zombie, the more bragging rights you have