Viruses contain some of the structures and exhibit some of the activities that are common to organic life, but they are missing many of the others. In general, viruses are entirely composed of a single strand of genetic information encased within a protein capsule. They lie somewhere between supra molecular complexes and very simple biological entities.
Viruses lack most of the internal structure and machinery which characterize 'life', including the biosynthetic machinery that is necessary for reproduction. In order for a virus to replicate it must infect a suitable host cell.
Viruses exist in two distinct states. When not in contact with a host cell, the virus remains entirely dormant and is called virion. During this time there are no internal biological activities occurring within the virus, and in essence the virus is no more than a static organic particle. They can remain in this dormant state for extended periods of time.
When the virion comes into contact with the appropriate host, it becomes active and displays properties typified by living organisms, such as reacting to its environment and directing its efforts toward self-replication.