Definition: a microscopic parasite that infects cells in biological organisms. they reproduce only by invading and controlling other cells as they lack the cellular machinery for self-reproduction.

A virus is a microscopic parasite that infects cells in biological organisms, they reproduce only by invading and controlling other cells as they lack the cellular machinery for self reproduction. Additionally, viruses contain at least one unique protein synthesized by specific genes in the nucleic acid, the proteins forms a shell called capsid around the nuclei acid.