The most useful and most widely used classification system distinguishes viruses according to the type of nucleic acid they use as genetic material and the viral replication method they employ to coax host cells into producing more viruses:
In addition virologists also study subviral particles, infectious entities even smaller than viruses: viroids (naked circular RNA molecules infecting plants), satellites (nucleic acid molecules with or without a capsid that require a helper virus for infection and reproduction), and prions (proteins that can exist in a pathological conformation that induces other prion molecules to assume that same conformation).
The latest report by the International Committee on Taxonomy of Viruses (2005) lists 5450 viruses, organized in over 2,000 species, 287 genera, 73 families and 3 orders.
It is of course possible that different alternatives apply to different virus groups.
Of particular interest here is mimivirus, a giant virus that infects amoebae and carries much of the molecular machinery traditionally associated with bacteria. Is it a simplified version of a parasitic prokaryote, or did it originate as a simpler virus that acquired genes from its host?
The evolution of viruses, which often occurs in concert with the evolution of their hosts, is studied in the field of viral evolution.
While viruses reproduce and evolve, they don't engage in metabolism and depend on a host cell for reproduction. The often-debated question of whether they are alive or not is a matter of definition that does not affect the biological reality of viruses.
When the immune system of a vertebrate encounters a virus, it produces specific antibodies which bind to the virus and mark it for destruction. The presence of these antibodies is often used to determine whether a person has been exposed to a given virus in the past, with tests such as ELISA. Vaccinations protect against viral diseases, in part, by eliciting the production of antibodies. Specifically constructed monoclonal antibodies can also be used to detect the presence of viruses, with a technique called fluorescence microscopy.
A second defense of vertebrates against viruses, cell-mediated immunity, involves immune cells known as T cells: the body's cells constantly display short fragments of their proteins on the cell's surface, and if a T cell recognizes a suspicious viral fragment there, the host cell is destroyed and the virus-specific T-cells proliferate. This mechanism is jump-started by certain vaccinations.
RNA interference, an important cellular mechanism found in plants, animals and many other eukaryotes, most likely evolved as a defense against viruses. An elaborate machinery of interacting enzymes detects double-stranded RNA molecules (which occur as part of the life cycle of many viruses) and then proceeds to destroy all single-stranded versions of those detected RNA molecules.
Every lethal viral disease presents a paradox: killing its host is obviously of no benefit to the virus, so how and why did it evolve to do so? Today it is believed that most viruses are relatively benign in their natural hosts; the lethal viral diseases are explained as resulting from an "accidental" jump of the virus from a species in which it is benign to a new one that is not accustomed to it (see zoonosis). For example, serious influenza viruses probably have pigs or birds as their natural host, and HIV is thought to derive from the benign monkey virus SIV.
While it has been possible to prevent (certain) viral diseases by vaccination for a long time, the development of antiviral drugs to treat viral diseases is a comparatively recent development. The first such drug was interferon, a substance that is naturally produced by certain immune cells when an infection is detected and stimulates other parts of the immune system.
Growing animal viruses outside of the living host animal is more difficult. Classically, fertilized chicken eggs have often been used, but cell cultures are increasingly employed for this purpose today.
Since viruses that infect eukaryotes need to transport their genetic material into the host cell's nucleus, they are attractive tools for introducing new genes into the host (known as transformation or transfection). Modified retroviruses are often used for this purpose, as they integrate their genes into the host's chromosomes.
This approach of using viruses as gene vectors is being pursued in the gene therapy of genetic diseases. An obvious problem to be overcome in viral gene therapy is the rejection of the transforming virus by the immune system.
Oncolytic viruses are viruses that preferably infect cancer cells. While early efforts to employ these viruses in the therapy of cancer failed, there have been reports in 2005 and 2006 of encouraging preliminary results.
In 1892 Dimitri Ivanovski showed that a disease of tobacco plants, tobacco mosaic disease, could be transmitted by extracts that were passed through filters fine enough to exclude even the smallest known bacteria. In 1898 Martinus Beijerinck, also working on tobacco plants, found that this "filterable agent" grew in the host and was thus not a mere toxin. The question of whether the agent was a "living fluid" or a particle was however still open.
In 1903 it was suggested for the first time that transduction by viruses might cause cancer. Such an oncovirus in chickens was described by Francis Peyton Rous in 1911; it was later called Rous sarcoma virus 1 and understood to be a retrovirus. Several other cancer-causing retroviruses have since been described.
The existence of viruses that infect bacteria was first recognized by Frederick Twort in 1911, and, independently, by Felix d'Herelle in 1917. Since bacteria could be grown easily in culture, this led to an explosion of virology research. An important investigator in this area, Max Delbrück, described the basic life cycle of a virus in 1937: rather than "growing", a virus particle is assembled from its constituent pieces in one step; eventually it leaves the host cell to infect other cells. The Hershey-Chase experiment in 1952 showed that only DNA and not protein enters a bacterial cell upon infection with bacteriophage T2. Transduction of bacteria by bacteriophages was first described in the same year.
While plant viruses and bacteriophages can be grown comparatively easily, animal viruses normally require a living host animal, which complicates their study immensely. In 1931 it was shown that influenza virus could be grown in fertilized chicken eggs, a method that is still used today to produce vaccines. In 1937, Max Theiler managed to grow the yellow fever virus in chicken eggs and produced a vaccine from an attenuated virus strain; this vaccine saved millions of lives and is still being used today.
In 1949 John F. Enders, Thomas Weller and Frederick Robbins reported that they had been able to grow poliovirus in cultured human embryonal cells, the first significant example of an animal virus grown outside of animals or chicken eggs. This work aided Jonas Salk in deriving a polio vaccine from killed polio viruses; this vaccine was shown to be effective in 1955.
The first virus that could be crystalized and whose structure could therefore be elucidated in detail was tobacco mosaic virus (TMV), the virus that had been studied earlier by Ivanovski and Beijerink. In 1935, Wendell Stanley achieved its crystallization for electron microscopy and showed that it remains active even after crystallization. Clear X-ray diffraction pictures of the crystallized virus were obtained by Bernal and Fankuchen in 1941. Based on such pictures, Rosalind Franklin proposed the full structure of the tobacco mosaic virus in 1955. Also in 1955, Heinz Fraenkel-Conrat and Robley Williams showed that purified TMV RNA and its capsid (coat) protein can assemble by themselves to form functional viruses, suggesting that this simple mechanism is likely the natural assembly mechanism within the host cell.
In 1965, Howard Temin described the first retrovirus: an RNA-virus that was able to insert its genome in the form of DNA into the host's genome. Reverse transcriptase, the key enzyme that retroviruses use to translate their RNA into DNA, was first described in 1970, independently by Howard Temin and David Baltimore. The first retrovirus infecting humans was identified by Robert Gallo in 1974. Later it was found that reverse transcriptase is not specific to retroviruses; retrotransposons which code for reverse transcriptase are abundant in the genomes of all eukaryotes. About 10-40% of the human genome derives from such retrotransposons.
In 1975 the functioning of oncoviruses was clarified considerably. Until that time, it was thought that these viruses carried certain genes called oncogenes which, when inserted into the host's genome, would cause cancer. Michael Bishop and Harold Varmus showed that the oncogene of Rous sarcoma virus is in fact not specific to the virus but is contained in healthy animals of many species. The oncovirus can switch this pre-existing benign proto-oncogene on, turning it into a true oncogene that causes cancer.
1976 saw the first recorded outbreak of Ebola hemorrhagic fever, a highly lethal virally transmitted disease.
In 1977, Frederick Sanger achieved the first complete sequencing of the genome of any organism, the bacteriophage Phi X 174. In the same year, Richard Roberts and Phillip Sharp independently showed that the genes of adenovirus contain introns and therefore require gene splicing. It was later realized that almost all genes of eukaryotes have introns as well.
The first cases of AIDS were reported in 1981, and HIV, the retrovirus causing it, was identified in 1983 by Robert Gallo and Luc Montagnier. Tests detecting HIV infection by detecting the presence of HIV antibody were developed. Subsequent tremendous research efforts turned HIV into the best studied virus. Human Herpes Virus 8, the cause of Kaposi's sarcoma which is often seen in AIDS patients, was identified in 1994. Several antiretroviral drugs were developed in the late 1990s, decreasing AIDS mortality dramatically in developed countries. The Hepatitis C virus was identified using novel molecular cloning techniques in 1987, leading to screening tests that dramatically reduced the incidence of post-transfusion hepatitis.
The first attempts at gene therapy involving viral vectors began in the early 1980s, when retroviruses were developed that could insert a foreign gene into the host's genome. They contained the foreign gene but did not contain the viral genome and therefore could not reproduce. Tests in mice were followed by tests in humans, beginning in 1989. The first human studies attempted to correct the genetic disease severe combined immunodeficiency (SCID), but clinical success was limited. In the period from 1990 to 1995, gene therapy was tried on several other diseases and with different viral vectors, but it became clear that the initially high expectations were overstated. In 1999 a further setback occurred when 18-year-old Jesse Gelsinger died in a gene therapy trial. He suffered a severe immune response after having received an adenovirus vector. Success in the gene therapy of two cases of X-linked SCID was reported in 2000.
In 2002 it was reported that poliovirus had been synthetically assembled in the laboratory, representing the first synthetic organism. Assembling the 7741-base genome from scratch, starting with the virus's published RNA sequence, took about two years. In 2003 a faster method was shown to assemble the 5386-base genome of the bacteriophage Phi X 174 in 2 weeks.
The strain of Influenza A virus subtype H1N1 that killed up to 50 million people during the Spanish flu pandemic in 1918 was reconstructed in 2005. Sequence information was pieced together from preserved tissue samples of flu victims; viable virus was then synthesized from this sequence.
In 2006 and 2007 it was reported that introducing a small number of specific transcription factor genes into normal skin cells of mice or humans can turn these cells into pluripotent stem cells, known as Induced Pluripotent Stem Cells. The technique uses modified retroviruses to transform the cells; this is a potential problem for human therapy since these viruses integrate their genes at a random location in the host's genome, which can interrupt other genes and potentially causes cancer.