Virus

A virus is a microscopic organism that can replicate only inside the cells of a host organism. Most viruses are so tiny they are only observable with at least a conventional optical microscope. Viruses infect all types of organisms, including animals and plants, as well as bacteria and archaea. Approximately 5000 different viruses have been described in detail at the current time, although it is known that there are millions of distinct types.^[1]  Viruses are found in virtually every ecosystem on Earth, and these minute life forms are thought to be the most abundant type of biological entity.^[2]  The study of viruses is known as virology, a specialty within the field of  microbiology. 
The common concept of viruses focuses on their role as pathogen. Actually, there are vast numbers of viral entities that are beneficial to individual species as well as providing ecosystem services. For example, a class of viruses known as bacteriophages can kill a spectrum of harmful bacteria, providing protection to  humans as well as other biota.
Viruses are key in the carbon cycle; their role in ocean biochemistry includes microbological metabolic—including decomposition—processes. It is this decomposition that stimulates massive carbon dioxide respiration of marine flora. That respiration annililates effectively about three gigatons of carbon each year from the atmosphere. Significantly, viruses are being developed as tools for constructive modern medicine as well as the critical field of nanotechnology.
Unlike prions and viroids, viruses consist of two or three parts: a helical molecule, protein coat and sometimes a viral wrapper. All viruses have genes constructed from either Deoxyribonucleic acid (DNA) or Ribonucleic acid (RNA)—long helical molecules that carry genetic information. All viruses have a protein coat that protects these genes, and some are wrapped in a viral envelope of fat that surrounds them when they are outside a cell. (Viroids do not have a protein coat and prions contain neither RNA nor DNA).
Viruses vary from simple helical and icosahedral shapes to more complex structures. Most viruses are approximately one hundred times smaller than an average bacterium. The origins of viruses in the evolutionary history of life are unclear. Some may have evolved from plasmids—fragments of DNA that can migrate between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity.

Lifeform or not?

Viruses have no ability to metabolize on their own, but depend upon a host organism for replication and manufacture of chemicals needed for such replication. Rybicki has characterized viruses as a form "at the edge of life".^[3]  Viruses are found in  Modern taxonomy and that taxonomy considers viruses as a totally separate form of life from cellular organisms—and some would say that they are merely complex molecules with a protein coating and not a lifeform at all. Since viruses are capable of self replication, they are clearly some type of lifeform, and likely involved with the early evolutionary development of such other simple lifeforms as bacteria and protists.
Viruses differ, however, from the simpler autonomous replication of chemical crystals. This is since a virus can inherit a genetic mutation and is also subject to similar natural selection processes of cellular organisms. A virus cannot be labelled simply, therefore, as inanimate or lifeless. Here, we consider it a lifeform, but we adhere to current taxonomy and do not credit it with a parallel domain to other recognized cellular lifeforms.  

Evolution

Although there is no detailed catalogue of the evolutionary relationships of viruses and hosts, certain gerneral characterisations can be made. In some such viral groups as poxviruses, papillomaviruses and tobamoviruses, molecular taxonomy aligns generally with the genetic relationships of their hosts.^[4] This suggests that the affilations of those viral groups predate their present derivatives, and, in fact, that these three viral groups and their hosts likely co-evolved. There are clear examples where an otherwise genetically close group like the tobamoviruses include a genetiically outlying host; in particular, the tobamoviruses generally utilize plants of the Solanaceae family, but an orchid and a cactus virus can also be found in the group.
Recombination of genome parts of viruses poses a more vexing puzzle, since the events are virtually random pieces of an evolutionary chain. Retroviruses and luteoviruses are examples of viral groups where large numbers of recombinations have occurred to produce new organisms. Sometimes these produced genome splices occur naturally using fragments that are either viral or cellular in nature. In some cases the product is more of a re-arrangement of genomic parts—referred to as psuedo-recombination. The Western Equine Encephalovirus is a known example of this last category.
It is likely that viruses began host relationships with archaea and bacteria about two billion years ago; it has been suggested, however, that the proliferation of terrestrial vascular plants was the watershed event in evolution that enabled the explosion of numbers of viral organisms and pathways.^[5]

Taxonomy

Tobacco Mosaic Virus schematic diagram. Source: Univ. Wisconsin There are two complementary systems for viral taxonomy: the ICTV and Baltimore approaches. In the case of the ICTV taxonomy, there are five distinct orders: Caudovirales, Herpesvirales, Mononegavirales, Nidovirales, and Picornavirales. Within that hierarchy reside 82 families, 307 genera, 2083 species.^[6]
David Baltimore devised an earlier system based on the method of viral messenger RNA synthesis.^[7] The Baltimore scheme is founded on the mechanism of  messenger RNA production.  Although viruses must replicate mRNAs from their genomes to produce proteins and reproduce, distinctly different mechanisms are employed within each viral family. Viral genomes may be single ((ss) or double-stranded (ds), may be RNA or DNA based, and may optionally employ reverse transcriptase (RT); furthermore single strand RNA virus helices may be either sense (+) or antisense (−). These nuances divide viruses into seven Baltimore groups.
This Baltimore classification of scheme is centered around the concept of messenger RNA replication, since viruses  generate messenger RNA from their genomic coding to produce proteins and thence replicate themselves. The resulting Baltimore groups are:

• I: dsDNA type (examples: Adenovirus, Herpesvirus, Poxvirus)
• II: ssDNA type (+)sense DNA (example: Parvovirus)
• III: dsRNA type (example: Reovirus)
• IV: (+)ssRNA type (+)sense RNA (examples: Picornavirus, Togavirus)
• V: (−)ssRNA type (−)sense RNA (examples: Orthomyxovirus, Rhabdovirus)
• VI: ssRNA-RT type (+)sense RNA with DNA intermediate to life-cycle (example: Retrovirus)
• VII: dsDNA-RT type (example: Hepadnavirus)