More than any other aspect biology, the existence of viruses challenges our fundamental notions of what it means for an organism to be alive.
Alone, the virus is little more than a dead strand of dna or rna, held together in a casing.
They don’t have any metabolic activity of their own but can become animated by the metabolic activity of another organism, incorporating themselves into the hosts processes.
There is something odd about the viral life cycle though. It is basically backwards.
Whereas all other organisms start out as something living, and become dead, the virus starts out as something dead, and becomes living.
But when all other organisms reproduce, they give birth to something living, while the virus does not. The virus gives birth to something dead.
Now one might think that what’s different about the virus is the fact that it requires a host to propagate, but while parasites have this in common, they do not share this backwards life cycle. The parasite gives birth to something living, that later becomes dead, like any other organism, but it does not reproduce into a dead organism that later comes to life only in the presence of another.
Now perhaps it is akin to the state of dormancy bacteria sometimes enter into. But even this is not quite the same, as metabolic processes do not simply stop in this hibernation, but only slow down tremendously. But in the dead virus there is none. Were any other organism to cease all metabolic activity so utterly and completely, and for such long periods, we would have to call it dead, except they find a way to come alive again.