Should the play be called dead?

[DS]

I have a question regarding wheter a play can be called dead if an NFL player kneels without another player touching him. In college football a play is called dead once the player with possesion of the ball has a part of the body, other than his hand, on the ground. However in the NFL a player has to first be touched, even though he is on the ground, for the whistle to be blown. I was wondering if there is ANY scenario (other than a fair catch call or a touchback) that allows a player to kneel and have the whistle blown. What if a player is hurt? Can that player kneel and the play called dead? Or if a player slides (other than a quarterback, because then the play is called dead) and is untouched for a certain number of seconds will the whistle blow to signify the play has ended?
I thought after a number of seconds that a play would be called dead if a player intentionally kneels or slides, am I wrong?