In what sense are parasitic bacteria that depend on the host for many important functions self-sustaining while viruses are not?
As I said, you have to consider the system of parasite+host (plus any other supporting processes).
I think a lot of the confusion arises from people confusing objects with processes that unfold over time. You can't ask if an object is alive by itself; you have to specify the time-dynamics of the system. Statements like 'a bacterium is alive' are problematic because a frozen bacterium in a block of ice is definitely not alive. Similarly, a virus that is dormant is most definitely not alive. But that same virus inside a living host cell is participating in a ...
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