A virus is an infectious agent that straddles the line between “living” and “non-living”. In fact it is commonly described as non-living due to its lack of key characteristics of living things, such as the fact that it can only replicate inside the living cells of other organisms.
Viruses are composed of an outer shell, usually composed of proteins, and a genome – either a DNA or RNA molecule that contains the instructions necessary to make more viruses. Viruses infect their target cells by using their viral “coat” proteins to help them attach to a host cell, and then to inject their genome (usually DNA) into the cell. This genome then recruits the host cell’s enzymes to express the genes encoded by the viral genome to produce more “coat” proteins, and to make more copies of the viral genome.
Because, viruses do not have the necessary enzymes to replicate themselves, they use the infected cell to accomplish this. Similar to a photocopier, one virus acts as a template to make hundreds or thousands of exact copies which can then be released and spread to other cells nearby.