Viruses are considered non-living because they only exhibit characteristics of living organisms when they are occupying a host organism. A virus consists of a single strand of DNA or RNA enclosed in a protein capsule. They lack the internal structures needed for metabolic processes. When not in a host cell they are dormant and have no biological activity. Dormant viruses are called virions. Once a dormant virus comes in contact with a host cell in becomes an active virus and replicates using structures of the host cell. It has the ability to pass its genetic information on to future generations, which is a characteristic of living organisms, but it is unable to do so without a host cell. Virions can remain dormant for very long periods of time.