Answer:
Because they can only reproduce in a living cell
Step-by-step explanation:
Virus is an organism that is incapable of replicating on its own without a living host. A virus infects a living host and makes use of Its replicational ability to produce new virions. A virus consists of a genetic material surrounded by a Protein called CAPSID. It does not possess the characteristics of other living organisms.
This means that it does not carry out any of the features a living cell undergoes e.g reproduction, respiration etc. Due to this reason, it is not classified as a living thing. They don't possess organnelles that carry out this peculiar functions.
A virus is considered a peculiar microorganism because it can only live and reproduce in a living host cell.