"I was thinking a mentally-debilitating disease, something like syphilis," William admitted. "If it's not possession, then it's something turning their brains into Swiss cheese. And once the focus is gone, just like with patients at St. Mungo's, the magic use is essentially gone as well. The potential ability itself is still there, it just can't be channeled anymore."
He hung back to seal up a crack at the bottom of the shallow flight of stairs leading toward the dormitories - no reason for anyone else to know where that went - and called ahead to Pete, "Another staircase on your right, pull the wall sconce toward you and it should open up. You can seal the door from there, or it leads to the dungeons right outside Slytherin, if you want to use it."
He wasn't insinuating anything at all by that; Pete had already expressed his desire to go there. And it was far more convenient than fighting through the public hallways.