Scientists do not agree on whether viruses are living systems or not, leading some people to call them non-living. According the Science Education Resource Center at Carleton College, viruses straddle the line between living and non-living entities. While viruses act similarly to bacteria in some ways, such as causing disease, they are much smaller than bacteria and do not exhibit all of the characteristics of living organisms.
Viruses cannot perform most basic life functions unless they are inside a living cell. Science Education Resource Center at Carleton College explains that viruses exist in two distinct states. When not invading a cell, viruses are essentially dormant and are called virions by scientists. Virions can remain in this inanimate state for an extended period of time, only becoming active when they contact a suitable host cell. At this time, the virus begins interacting with its external environment and engaging in self-replication.
Science Daily explains that viruses are simple entities, comprised of only a small amount of genetic material and a protein coat that encapsulates the genetic material. Viruses infect a wide variety of organisms, including bacteria, plants, fungi and animals. Viruses cause some of the most serious diseases that affect humans, such as HIV, Ebola and SARS.