You start off in [x]*, and a disease is spawned somewhere within in [x].
The host can pick the mode of what the disease is, and each disease has a different effect of what it does to the player: turning them into a mindless zombie, killing them right away, making them walk slower and slowly dying from being exposed to too much of the disease, the possibilites are endless.
The disease will -slowly- go through doors, but otherwise it moves at a decent rate, giving you a good chance to get away from it.
*I don't know, a lab, a ship, who knows?
Sound good? :|
The disease will always be visible.