Question

Viruses are kept at the boundary of living and mom living world why?

Answer 1

Virus is considered a borderline between living and non living because it has features similar to both. Outside a living body a virus can be crystallized and appears to be non living as it cannot carry any meatbolic activities. But once it enters a suitable host it activates its genetic material and is able to carry out all the activities using the cellular machinery of the host cell. 

Answer 2

Viruses are the microorganisms which are still not classified under the taxonomical hierarchy.
They are non living outside the host cell/any living cell whereas whenever they invade a living host cell they utilise the host's genetic material to multiply and produce more in no.
Hence they behave as living inside another living cell
So viruses are a boundary between living and non living
Hope that helps you :-)