Non-cellular life is life which exists without a cell structure. Until the 21st century, people generally accepted that in order to be considered a life form, an organism had to have a cell, although cell structure could vary considerably. This excluded things like viruses from a list of "living" organisms. Additional research, however, suggested that this classification might be erroneous, and that things such as viruses could actually be considered life forms. This gave rise to the term "non-cellular life" to describe such organisms, differentiating them from cellular life like bacteria, fungi, archaea, protists, animals, and plants.
While people had suggested that viruses had many traits associated with life, it wasn't until 2003 that researchers discovered a virus with protein-making capabilities. The ability to synthesize proteins is considered a major factor in determining whether or not an organism is alive, and most viruses must hijack cells in order to build proteins. The fact that the Mimivirus can create proteins led to the realization that there might be more to viruses than meets the eye.
Given the large number of as-yet undiscovered viruses in the world, it is entirely possible that researchers may one day find more viruses capable of producing proteins. This would suggest that viruses perhaps evolved from earlier life forms which were capable of producing proteins independently of a cell. It also broadens the horizons considerably when thinking about extraterrestrial life; if non-cellular life exists on Earth, it may be found elsewhere as well.
In addition to viruses, structures such as cosmids, satellites, viriods, fosmids, prions, phagemids, and transposons may also be considered non-cellular life. Some scientists refer to non-cellular life as Acytota or Aphanobiota, and cellular life as Cytota. If the concept of non-cellular life grows to be more widely accepted, Acyota or Aphanobiota may be added to the three-domain system in taxonomy, turning it into a four-domain system which can be used to classify all forms of life on Earth.
Recognition of viruses as a legitimate life form may also deepen human understanding of these fascinating and complex organisms. Like cellular life forms, viruses clearly have their own agenda and life goals, although these goals may conflict with human, animal, and plant interests at times. The fact that some viruses are capable of producing proteins could also become a factor in treatment of certain viral infections, as medications could potentially be devised to target these proteins and eradicate the virus.