We don't. And if it doesn't work, you're out of what you had, and I...

[Might end up even worse off.]

Katara, this isn't something we can risk being wrong on. If I bite...even scratch one person in my condition, it passes on to them, too. If I die, I come back as a full vampire. And there are people who might need what you have...and that it will work on without hoping that's what it'll do.