Question

Why are the virus considered as being at the border line between living and nonliving thing?

Answer 1

Virus don't have any mechanisms of their own. So when they enter a living body(host) they use the mechanisms of the host to carry out their own life processes. Hence they are almost like a non living thing when they are outside a host as they cannot carry out any life processes independently. But when they enter the host they use the host's mechanisms, carry out life processes and are considered as living.