Yes, but you are defining a very narrow definition of "living". You might keep an intestinal parasite alive for a while without a live host, but it would not reproduce. The species would die out in one generation. There are other constraints on reproduction. Passenger pigeons died out when their numbers declined. For some reason that species required a critical minimum number of individuals in order to survive. Something to think about when you are tempted to wonder how evolution can select for group benefit rather than individual benefit.
Parasitism exists. Parasites cannot survive and reproduce without hosts. My original question is, how is this conceptually different from the need of a virus or prion for a host in order to reproduce? It is true that prions and viruses do not metabolize, but they take over the metabolism of their host and bend it to their own reproduction. And they evolve.
I think any definition of life should address this behavior.
How is a parasite's requirement for food different from other organisms?
Why wouldn't they be able to reproduce?
And anyway I say that you don't have to be able to reproduce to be classified as living. Mules are alive. You do agree with that, right?
For some reason [Passenger pigeons] required a critical minimum number of individuals in order to survive.
Are saying that Passenger pigeons weren't alive because they went extinct?
My original question is, how is [Parasitism] conceptually different from the need of a virus or prion for a host in order to reproduce?
I didn't say viruses and prions aren't parasites, just that they're not alive.
It is true that prions and viruses do not metabolize, but they take over the metabolism of their host and bend it to their own reproduction.
I think it is more interesting to think of them as epiphenomena of DNA-based life.
And they evolve.
Yes but that is a characteristic of populations. It is the individuals that are alive - or not in the case of prions and viruses. As with reproductive capacity, "evolvability" is neither sufficient nor necessary to be classified as living. If some kind of cell were engineered to reproduce without any error whatsoever, we would still classify it as living.