A virus is a microorganism smaller than a bacteria, which cannot grow or reproduce apart from a living cell. A virus invades the living host cells and uses their chemical machinery to keep itself alive and to replicate itself. A virus acts in a parasitic fashion, using living cells to sustain itself. A virus may reproduce with fidelity or with mutations. This ability of the virus to mutate is responsible for some viruses to change slightly in each infected person, making treatment more difficult.
A Herpes Virus