Prions dont really have a metobolism,
so I would not call them life.

A virus is a lifeform to me. It cant live by its own, but thats not a requisite of life.
(humans cant live without certain bacterias in their digestion tract either)

I think the definition is more usefull to determine if a planet bears life at all, If a certain
structure is found on an extraterretrical planet.