Viruses are the smallest of pathogens

Viruses were discovered alongside disease-causing bacteria. It was thought that viruses were just smaller versions of bacteria and fell under the general umbrella of 'alive'. This however was disproved in 1935 when viruses lack of metabolism was unveiled.

Viruses exist in two states. When not replicating within a host cell, the virus remains entirely dormant. There are no biological changes occurring within the virus, and in essence the virus is no more than a static lump of RNA or DNA. In this simple, dormant status viruses are referred to as 'virions'. Virions can remain in this abiotic state as long as the organism is alive or an extended amount of time outside the body; waiting patiently to come out of dormancy. When the virion comes into contact with the appropriate host, it actively possesses cells and is referred to as a virus.