In Dungeon World, the cleric gets the 5th level spell Contagion, which states:
While this spell is ongoing a target you can see suffers from the disease of your choice and you take -1 to Cast a Spell.
I have a basic understanding of Dungeon World, and have read the book through aside from the Advanced Delving chapter and many of the monster entries. I understand how moves work and I understand that everything follows the fiction and comes from the fiction. I also understand that many discoveries about the world in DW come from the fiction as needed.
Does this spell basically mean that, if no diseases have been established, the cleric gets to make up a disease on the spot? Does the cleric get to determine what effects that disease has, or do diseases roughly translate to debilities? Might a disease cause several debilities? (Probably). And finally, making an assumption here, how can the GM rein in the "created" disease to ensure that it isn't an "instant win" spell for the cleric?
Hypothetically, the cleric says, "I give the goblin the instant black blood death disease which kills every infected creature within 5 seconds of contracting it." What happens then?
This is probably okay when framed in the context of DW (I get it, I do), but having not yet played a session I guess I'm having difficulty fully understanding how this type of situation doesn't "ruin" the game.