Virology is the study of viruses and virus-like agents: their structure, classification and evolution, their ways to infect and exploit cells for virus reproduction, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy. Virology is often considered a part of
microbiologyor of pathology.
Virus structure and classification
A major branch of virology is
virus classification. Viruses can be classified according to the host cell they infect: animal viruses, plant viruses, fungal viruses, and bacteriophages (viruses infecting bacteria, which include the most complex viruses). Another classification uses the geometrical shape of their capsid(often a helixor an icosahedron) or the virus's structure (e.g. presence or absence of a lipidenvelope). Viruses range in size from about 30 nm to about 450 nm, which means that most of them cannot be seen with light microscopes. The shape and structure of viruses can be studied with electron microscopy, with NMR spectroscopy, and most importantly with X-ray crystallography.
The most useful and most widely used classification system distinguishes viruses according to the type of
nucleic acidthey use as genetic material and the viral replicationmethod they employ to coax host cells into producing more viruses:
DNA viruses (divided into double-stranded DNA viruses and the much less common single-stranded DNA viruses),
RNA viruses (divided into positive-sense single-stranded RNA viruses, negative-sense single-stranded RNA viruses and the much less common double-stranded RNA viruses),
* reverse transcribing viruses (double-stranded reverse-transcribing DNA viruses and single-stranded reverse-transcribing RNA viruses including
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.
The taxa in virology are not necessarily
monophyletic. In fact, the evolutionary relationships of the various virus groups remain unclear, and three hypotheses regarding their origin exist:
# Viruses arose from non-living matter, separately from and in parallel to other life forms, possibly in the form of self-reproducing
RNA ribozymes similar to viroids.
# Viruses arose from earlier, more competent cellular life forms that became parasites to host cells and subsequently lost most of their functionality; examples of such tiny parasitic prokaryotes are
# Viruses arose as parts of the genome of cells, most likely
transposons or plasmids, that acquired the ability to "break free" from the host cell and infect other cells.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
While viruses reproduce and evolve, they don't engage in
metabolismand 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.
Viral diseases and host defenses
One main motivation for the study of viruses is the fact that they cause many important infectious diseases, among them the
common cold, influenza, rabies, measles, many forms of diarrhea, hepatitis, yellow fever, polio, smallpoxand AIDS. Some viruses, known as oncoviruses, contribute to certain forms of cancer; the best studied example is the association between Human papillomavirusand cervical cancer: it is now acknowledged that almost all cases of cervical cancer are caused by certain strains of this sexually transmitted virus. Some subviral particles also cause disease: Kuru and Creutzfeldt-Jakob diseaseare caused by prions, and hepatitis Dis due to a satellite virus.
The study of the manner in which viruses cause disease is
viral pathogenesis. The degree to which a virus causes disease is its virulence.
immune systemof a vertebrateencounters 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 antibodiescan 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 HIVis 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.
Molecular biology research and viral therapy
Bacteriophages, the viruses which infect bacteria, can be relatively easily grown as viral plaques on bacterial cultures. Bacteriophages occasionally move genetic material from one bacterial cell to another in a process known as transduction, and this horizontal gene transferis one reason why they served as a major research tool in the early development of molecular biology. The genetic code, the function of ribozymes, the first recombinant DNAand early genetic libraries were all arrived at using bacteriophages. Certain genetic elements derived from viruses, such as highly effective promoters, are commonly used in molecular biology research today.
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 therapyof genetic diseases. An obvious problem to be overcome in viral gene therapy is the rejection of the transforming virus by the immune system. Phage therapy, the use of bacteriophages to combat bacterial diseases, was a popular research topic before the advent of antibioticsand has recently seen renewed interest. Oncolytic viruses are viruses that preferably infect cancercells. 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. [ [http://www.isracast.com/tech_news/260106_tech.aspx Viruses: The new cancer hunters] , IsraCast, 1 March 2006]
Other uses of viruses
A new application of genetically engineered viruses in
nanotechnologywas recently described; see Virus#Materials science and nanotechnology. For a use in mapping neuronssee Pseudorabies#Applications in Neuroscience.
History of virology
A very early form of vaccination known as
variolationwas developed several thousand years ago in China. It involved the application of materials from smallpoxsufferers in order to immunize others. In 1717 Lady Mary Wortley Montaguobserved the practice in Istanbuland attempted to popularize it in Britain, but encountered considerable resistance. In 1796 Edward Jennerdeveloped a much safer method, using cowpoxto successfully immunize a young boy against smallpox, and this practice was widely adopted. Vaccinations against other viral diseases followed, including the successful rabiesvaccination by Louis Pasteurin 1886. The nature of viruses however was not clear to these researchers.
Dimitri Ivanovskishowed 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 Rousin 1911; it was later called Rous sarcoma virus1 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 Twortin 1911, and, independently, by Felix d'Herellein 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 experimentin 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 viruscould be grown in fertilized chicken eggs, a method that is still used today to produce vaccines. In 1937, Max Theilermanaged 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.
John F. Enders, Thomas Weller and Frederick Robbinsreported that they had been able to grow poliovirusin cultured human embryonal cells, the first significant example of an animal virus grown outside of animals or chicken eggs. This work aided Jonas Salkin 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 diffractionpictures of the crystallized virus were obtained by Bernal and Fankuchen in 1941. Based on such pictures, Rosalind Franklinproposed the full structure of the tobacco mosaic virus in 1955. Also in 1955, Heinz Fraenkel-Conratand Robley Williamsshowed that purified TMV RNAand 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 1963, the Hepatitis B virus was discovered by
Baruch Blumbergwho went on to develop a vaccine against Hepatitis B.
Howard Temindescribed 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 Galloin 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 Varmusshowed that the oncogene of Rous sarcoma virusis 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.
Frederick Sangerachieved the first complete sequencing of the genomeof any organism, the bacteriophage Phi X 174. In the same year, Richard Roberts and Phillip Sharp independently showed that the genes of adenoviruscontain introns and therefore require gene splicing. It was later realized that almost all genes of eukaryotes have introns as well.
A world-wide vaccination campaign led by the UN
World Health Organizationresulted in the eradication of smallpoxin 1979.
Stanley Prusinerdiscovered prions and showed that they cause scrapie.
The first cases of AIDS were reported in 1981, and
HIV, the retrovirus causing it, was identified in 1983 by Robert Galloand 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 sarcomawhich 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 viruswas identified using novel molecular cloningtechniques in 1987, leading to screening tests that dramatically reduced the incidence of post-transfusion hepatitis. [ [http://www.laskerfoundation.org/awards/library/2000c_cit.shtml 2000 Albert Lasker Award for Clinical Medical Research] , The Lasker Foundation. Accessed 20 February 2008]
The first attempts at
gene therapyinvolving 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 Gelsingerdied in a gene therapy trial. He suffered a severe immune response after having received an adenovirusvector. Success in the gene therapy of two cases of X-linked SCID was reported in 2000. [Zeger Debyser. [http://www.ohsu.edu/nod/documents/week1/short.vectorology.pdf A Short Course on Virology / Vectorology / Gene Therapy] , "Current Gene Therapy", 2003, 3, 495-499]
In 2002 it was reported that
poliovirushad 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 174in 2 weeks.
mimivirus, in some sense an intermediate between tiny prokaryotes and ordinary viruses, was described in 2003 and sequenced in 2004.
The strain of
Influenza A virus subtype H1N1that killed up to 50 million people during the Spanish flupandemic 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. [Cite news
issn = 0362-4331
last = Kolata
first = Gina
title = Experts Unlock Clues to Spread of 1918 Flu Virus
work = The New York Times
accessdate = 2008-02-03
date = 2005-10-06
url = http://www.nytimes.com/2005/10/06/health/06flu.html?pagewanted=2]
Harald zur Hausenhad shown that two strains of Human papillomavirus(HPV) cause most cases of cervical cancer. Two vaccines protecting against these strains were released in 2006.
In 2006 and 2007 it was reported that introducing a small number of specific
transcription factorgenes 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. [ [http://www.sciam.com/article.cfm?id=stem-cells-without-cancer Stem Cells—This Time without the Cancer] , Scientific American News, 30 November 2007]
Introduction to virus
List of viruses
*List of viral diseases
*Important publications in virology
*Villarreal, L. P. (2005) "Viruses and the Evolution of Life". ASM Press, Washington DC ISBN 1-55581-309-7
* Samuel Baron (ed.) (1996) "Medical Microbiology", 4th ed., [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.part.5437 Section 2: Virology] (freely searchable online book)
* Coffin, Hughes, Varmus. (1997) " [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowTOC&rid=rv.TOC&depth=10 Retroviruses] " (freely searchable online book)
External links and sources
* David Sander: [http://www.virology.net/ All the Virology on the WWW] - collection of links, pictures, lecture notes
* [http://media.med.sc.edu/microbiology2007/ Online lectures in virology] University of South Carolina
* [http://www.microbiologybytes.com/introduction/introduction.html MicrobiologyBytes: Origins of Virology]
* [http://www.microbiologybytes.com/tutorials/Time/Machine.html MicrobiologyBytes: The Virology Time Machine]
* [http://medicine.wustl.edu/~virology/timeline.htm Timeline of the history of virology] , from the
Washington University in St. Louis.
* [http://virology-online.com Wong's Virology] .
* [http://www.vrc.nih.gov Vaccine Research Center (VRC)] - Information concerning vaccine research studies
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