Virus is a submicroscopic organism that can replicate only inside a living host cell.
The ligands present in the virus interact with cognate host cell receptors and penetrate the host cell.
Viruses use the host cell machinery to replicate their genome and make progeny virions.
Some RNA viruses carry the RNA polymerizing machinery in their genome.
Except these, all other viruses use the host cell DNA polymerases, translating machinery, etc. to make all the essential components required for multiplication.
The viral genome integrates into the host cell genome causing it to prompt the replication of viral genome.