Of the 6 or 7 criteria biologists usually use to define life, viruses often fall short. E.g. viruses do not have their own metabolism.
Everyone has their own definition though. I remember reading a novel where life was defined simply as heredity, mutation, and adaptation. The second definition makes more sense to me, and implicitly includes viruses.
The most terrifying thing about prions is that they are infectious but not life: they can't be "killed" in the normal sense
While viruses are generally not considered to be alive, they are considered to be instances of life, aren't they?