Neither do algae or fungi, and they're alive. Plants have organs, but no nervous system, and they're alive. Not conscious, though - probably...
That's a whole other conundrum.
Just because we can use some of their proteins to prep our immune system is not evidence of life.
No, that's not the evidence of life. The evidence is that they actively seek to reproduce their DNA at the expense of cells that are more complex than they are.
They do sometimes behave as if they were alive,
but what is "alive"?
Alive is resisting death.
Until you come up with a more universal definition, settle for the presence of DNA. That's something we know crystals (which replicate themselves) and weather systems (which sometimes act as if they were alive) do not have. It's possible that life on other planets perpetuates its own kind without DNA, but then we'll have to look for more obviously anti-entropy activity.
By the time we go exploring I hope to have a protocol to ascertain what constitutes being alive in place because we have little idea of what we might find, if we find anything at all.
Though there doesn't seem to be any great urgency, it's a good idea to reflect on the question.
Just because we are used to myriad life-forms on Earth doesn't necessarily mean it's the same everywhere else in the cosmos.
I think it's bound to be similar, given the materials available and the physical laws that govern their processes.
.Viruses are a good example, but who can say its the only example? We may find others.
Viruses may be extraterrestrial. Or the remnant of a more complex protozoan that was near-fatally damaged by radiation.
If we adopted "Anything that can be killed must be alive", we have a new definition.
I was using that tongue-in-cheek. While it may work, the discovery of what can be killed could get our space-explorers into a lot of trouble. First contact goes more smoothly if you haven't just murdered the chief's egg-bearer. Still, whatever resists being sampled should be left alone.
Better walk softly and carry an excellent tricorder.