"You don't understand," Theta said, shoving him into a chair. "It jumps bodies with a touch. It starts out in one, and as soon as you stop it there, it leaps into someone else. It controls them, uses thir skills. It can -become- them. Imagine a Superman with it inside him. Or Ironman. The Doctor? Loki? Wolverine? What about Jack Harkness? It's in Rose now, one of them. You have seventy-two hours before it can move again."