We've tried and tried to make this clear throughout the text, but we'll keep trying. The new (as of the proofing version) sections on the adventuring life and Gming common situations are supposed to lead to this.
Fundamentally, we tell the GM over and over that their job is to present a fantastic world. That means that they show the reasonable fictional outcomes of the players' actions. The moves are a special case of that.
As a GM, when the player says "I cure his disease" the first thing to ask is "how?" If their response makes no sense—if they say "I stare at him real hard"—the GM just portrays the world and nothing happens. If the players take an action that could, maybe, lead to it—like "I pray to my god, chanting and lighting incense"—you respond with a soft move (they're looking at you to find out what happens). Maybe you give them what they want with a cost ("Yur, god of darkness, heals you but you find that you're uncomfortable in the light now"). Maybe you show signs of impending doom ("The red blotches of infection go away, all except one. You feel better, but the last blotch throbs with potential.").
We've tried to make this really clear: if a move doesn't apply, the GM portrays the world honestly and maybe makes a move.