Why are viruses considered as being the borderline between living and non-living things?

Hint: The viruses are found everywhere. Around millions of viruses are present in the atmosphere. Viruses are the very minute infectious endoparasite which multiplies inside a living host. The host can be bacteria, archaea or humans and animals.

Complete answer:
To solve this question, we must know the structure and properties of viruses.
The virus is made up of a protein coat and genetic material which can be DNA or RNA and is helical in shape.
The Protein coat or the capsid- It surrounds and protects the genetic material.
The genetic material DNA or RNA- It lies inside the protein coat which codes for structural and functional protein.

Properties:
1.The virus remains inert and is not affected by the adverse conditions due to the protective capsid. So it is considered as non-living.
2.The viruses are acellular. It only multiplies when it enters a living host. It produces multiple copies of its genome inside the host cell and starts producing its proteins. These proteins are harmful for the host and ultimately the host dies. Hence it is considered as living.

Note: The genome replication of DNA virus takes place inside the nucleus of the hot cell whereas the RNA virus replicates in the cytoplasm of the host. The virus enters the cell of the host through the tail which binds to receptors present on the cell surface. Only the viral genome enters the host and the capsid is left behind outside. Some of the commonly found viruses are Tobacco mosaic virus, influenza virus, Herpes virus etc.