Question

why are viruses considered to be on the borderline of living and non-living?

Answer 1

Viruses are non-living when they are outside the host cell as they don't have any cellular machinery of their own. But when they are present inside the body of a host, they are living. They take over the host cell machinery to replicate themselves, killing the host. That's why they are considered to be on the borderline between living and non-living.