They are infectious agents that replicate inside a host's body.
They are neither living nor non-living.
Examples- adenovirus, Hepatitis C virus, etc.
They usually lack a cell wall. Instead are surrounded by capsid (a protective protein coating)
They contain either RNA or DNA as the genetic material.
They utilize the hosts' machinery to replicate and transcribe their genome.
The hypothesis of virus origin:
According to the progressive hypothesis, viruses are just extraordinary pieces of genetic information.
These genetic elements gained the power to move in and out of living cells independently.
According to the regressive hypothesis, viruses are reduced parasitic forms of complex ancestral organisms.
According to the virus-first theory, viruses were the first to evolve.
However, none of the above hypotheses is entirely correct. Their origin is intensely contested because of the tremendous diversity among viruses & therefore is challenging to be classified.
Bacteria are considered the oldest forms of life on Earth. Therefore, viruses are not the first form of life to appear on Earth.