The video would have to be reviewed before receiving said ranking.
I couldn't imagine a lot of uploaders would abuse this as they wouldn't get paid.
Of course the question is, what's in it for Google? You've probably noticed when you search a disease, it provides a nice result with graphics and dialogs. Although I don't know if they'd be interesting in preserving that image over to Youtube, as that place is a cesspool for content creation, whatever that means...