Why are viruses considered to represent borderline between living and non-living?

Viruses are infectious agents with both living and nonliving characteristics. They can infect animals, plants, and even other microorganisms. Viruses that infect only bacteria are called bacteriophages and those that infect only fungi are termed mycophages. There are even some viruses called virophages that infect other viruses.

1. Living characteristics of viruses

a. They reproduce at a fantastic rate, but only in living host cells.

b. They can mutate.

2. Nonliving characteristics of viruses

a. They are acellular, that is, they contain no cytoplasm or cellular organelles.

b. They carry out no metabolism on their own and must replicate using the host cell's metabolic machinery. In other words, viruses don't grow and divide. Instead, new viral components are synthesized and assembled within the infected host cell.

c. The vast majority of viruses possess either DNA or RNA but not both.

Because of these characteristics, viruses are considered to represent borderline between living and non-living.

Recently, viruses have been declared as living entities based on the large number of protein folds encoded by viral genomes that are shared with the genomes of cells. This indicates that viruses likely arose from multiple ancient cells.

The vast majority of viruses contain only one type of nucleic acid: DNA or RNA, but not both. They are totally dependent on a host cell for replication. (They are strict intracellular parasites. Viral components must assemble into complete viruses (virions) to go from one host cell to another.