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.
Is it like...a gas??
In response to Sniper Joe
Yeah...different types of gasses though.
In response to Ol' Yeller
sounds cool but seems a bit far fetched if you use byonds tile based movement.
In response to Neophyte
Not to bright are we? Tile based movement would work just fine.
For X, how about Space Station 13? You should talk to the development team there and suggest Disease as a mode. SS13 would provide a basis and a lot of gameplay aside from the disease itself. In the context, SS13 could have different devices for curing, containing, stopping, etc, the disease to make gameplay all the more fun.