That's because I wasn't being clear. Is the matter/energy directly feeding the metabolism significantly less complex? Take the case of your parasite, say an intestinal parasite. Although it lives within a complex host, it feeds on matter of a very simple nature. In fact the host is optional - place the parasite in a broth of shredded proteins and other nutrients and it would do as well. And similarly would its host if the broth were directly delivered to the gut.
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.