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Virus

2007 Schools Wikipedia Selection. Related subjects: General Biology

       How to read a taxoboxViruses
   Herpes simplex virus 1 (HSV-1)
   Herpes simplex virus 1 (HSV-1)
           Virus classification

   Group: I - VII

                                   Groups

   I: dsDNA viruses
   II: ssDNA viruses
   III: dsRNA viruses
   IV: (+)ssRNA viruses
   V: (-)ssRNA viruses
   VI: ssRNA-RT viruses
   VII: dsDNA-RT viruses

   A virus is a microscopic particle (ranging in size from 20 - 300 n m)
   that can infect the cells of a biological organism. Viruses can
   replicate themselves only by infecting a host cell. They therefore
   cannot reproduce on their own. At the most basic level, viruses consist
   of genetic material contained within a protective protein coat called a
   capsid. They infect a wide variety of organisms: both eukaryotes (
   animals, yeasts, fungi, and plants) and prokaryotes (bacteria and
   archaea). A virus that infects bacteria is known as a bacteriophage,
   often shortened to phage. The word virus comes from the Latin, virus,
   "poison" (syn. venenum). The study of viruses is known as virology and
   people who study viruses are known as virologists. Viruses cause
   several serious human diseases, such as AIDS, influenza and rabies.
   Therapy is difficult for these diseases as antibiotics have no effect
   on viruses and few antiviral drugs are known. The best way to prevent
   viral diseases is with a vaccine, which produces immunity.

   It has been argued extensively whether viruses are living organisms.
   Most virologists consider them non-living, as they do not meet all the
   criteria of the generally accepted definition of life. They are similar
   to obligate intracellular parasites as they lack the means for
   self-reproduction outside a host cell, but unlike parasites, viruses
   are generally not considered to be true living organisms. A definitive
   answer is still elusive. Some organisms considered to be living exhibit
   characteristics of both living and non-living particles, as viruses do.
   For those who consider viruses living, viruses are an exception to the
   cell theory proposed by Theodor Schwann, as viruses are not made up of
   cells.

Discovery

   Computer-generated image of virions
   Computer-generated image of virions

   Viral diseases such as rabies have affected humans for many centuries.
   There is hieroglyphical evidence of Polio in the ancient Egyptian
   empire. However, the cause of these diseases was discovered relatively
   recently. In 1717, Mary Montagu, the wife of an English ambassador to
   the Ottoman Empire, observed local women inoculating their children
   against Smallpox. In the late 18th century, Edward Jenner observed and
   studied Miss Sarah Nelmes, a milkmaid who had previously caught Cowpox
   and was subsequently found to be immune to Smallpox, a similar, but
   devastating virus. Jenner developed the first vaccine, based on these
   findings, and smallpox is currently all but wiped out.

   In the late 19th century Charles Chamberland developed a porcelain
   filter. This filter was used to study the first documented virus,
   tobacco mosaic virus. Shortly afterwards, Dimitri Ivanovski published
   experiments showing that crushed leaf extracts of infected tobacco
   plants were still infectious even after filtering the bacteria from the
   solution. At about the same time, several others documented filterable
   disease-causing agents, with several independent experiments showing
   that viruses were different from bacteria, yet they could also cause
   disease in living organisms. These experiments showed that viruses are
   orders of magnitudes smaller than bacteria. The term virus was coined
   by the Dutch microbiologist Martinus Beijerinck.

   In the early 20th century, Frederick Twort discovered that bacteria
   could be attacked by viruses. Felix d'Herelle, working independently,
   showed that a preparation of viruses caused areas of cellular death on
   thin cell cultures spread on agar. Counting the dead areas allowed him
   to estimate the original number of viruses in the suspension. The
   invention of Electron microscopy provided the first look at viruses. In
   1935 Wendell Stanley crystallised the tobacco mosaic virus and found it
   to be mostly protein. A short time later the virus was separated into
   protein and nucleic acid parts.

Origins

   The origins of modern viruses are not entirely clear. It may be that no
   single mechanism can account for all viruses. They do not fossilize
   well, so molecular techniques have been the most useful means of
   hypothesising how they arose. Research in microfossil identification
   and molecular biology may yet discern fossil evidence dating to the
   Archean or Proterozoic eons. Two main hypotheses currently exist.
     * Small viruses with only a few genes may be runaway stretches of
       nucleic acid originating from the genome of a living organism.
       Their genetic material could have been derived from transferable
       genetic elements such as plasmids or transposons, which are prone
       to moving around, exiting, and entering genomes.

     * Viruses with larger genomes, such as poxviruses, may have once been
       small cells which parasitised larger host cells. Over time, genes
       not required by their parasitic lifestyle would have been lost in a
       streamlining process known as retrograde-evolution or
       reverse-evolution. The bacteria Rickettsia and Chlamydia are living
       cells that, like viruses, can only reproduce inside host cells.
       They lend credence to the streamlining hypothesis, as their
       parasitic lifestyle is likely to have caused the loss of genes that
       enabled them to survive outside a host cell.

   Other infectious particles which are even simpler in structure than
   viruses include viroids, satellites, and prions.

Classification

   In taxonomy, the classification of viruses is rather difficult due to
   the lack of a fossil record and the dispute over whether they are
   living or non-living. They do not fit easily into any of the domains of
   biological classification and therefore classification begins at the
   family rank. However, the domain name of Acytota has been suggested.
   This would place viruses on a par with the other domains of Eubacteria,
   Archaea, and Eukarya. Not all families are currently classified into
   orders, nor all genera classified into families.

   As an example of viral classification, the chicken pox virus belongs to
   family Herpesviridae, subfamily Alphaherpesvirinae and genus
   Varicellovirus. It remains unranked in terms of order. The general
   structure is as follows.

          Order (-virales)

                Family (-viridae)

                      Subfamily (-virinae)

                            Genus (-virus)

                                  Species (-virus)

   The International Committee on Taxonomy of Viruses (ICTV) developed the
   current classification system and put in place guidelines that put a
   greater weighting on certain virus properties in order to maintain
   family uniformity. In determining order, taxonomists should consider
   the type of nucleic acid present, whether the nucleic acid is single-
   or double-stranded, and the presence or absence of an envelope. After
   these three main properties, other characteristics can be considered:
   the type of host, the capsid shape, immunological properties and the
   type of disease it causes.

   In addition to this classification system, the Nobel Prize-winning
   biologist David Baltimore devised the Baltimore classification system.
   This places a virus into one of seven Groups, which distinguish viruses
   based on their mode of replication and genome type. The ICTV
   classification system is used in conjunction with the Baltimore
   classification system in modern virus classification.

Structure

   A complete virus particle, known as a virion, is little more than a
   gene transporter, consisting in its simplest form of nucleic acid
   surrounded by a protective coat of protein called a capsid. A capsid is
   composed of proteins encoded by the viral genome and its shape serves
   as the basis for morphological distinction. Virally coded protein
   subunits - sometimes called protomers - will self-assemble to form the
   capsid, generally requiring the presence of the virus genome - however,
   many complex viruses code for proteins which assist in the construction
   of their capsid. Proteins associated with nucleic acid are known as
   nucleoproteins, and the association of viral capsid proteins with viral
   nucleic acid is called a nucleocapsid.

   In general, there are four main morphological virus types:
   Image Helical viruses
   Diagram of a helical capsid
   Diagram of a helical capsid
   Helical capsids are composed of a single type of subunit stacked around
   a central axis to form a helical structure which may have a central
   cavity, or hollow tube. This arrangement results in rod-shaped or
   filamentous virions: these can be anything from short and highly rigid,
   to long and very flexible. The genetic material - generally
   single-stranded RNA, but also ssDNA in the case of certain phages - is
   bound into the protein helix, by charge interactions between the
   negatively-charged nucleic acid and positive charges on the protein.
   Overall, the length of a helical capsid is related to the length of the
   nucleic acid contained within it, while the diameter is dependent on
   the size and arrangement of protomers. The well-studied Tobacco mosaic
   virus is an example of a helical virus.
   Image Icosahedral viruses
   Electron micrograph of icosahedral virions
   Electron micrograph of icosahedral virions
   Icosahedral capsid symmetry results in a spherical appearance of
   viruses at low magnification but actually consists of capsomers
   arranged in a regular geometrical pattern, similar to a soccer ball,
   hence they are not truly "spherical". Capsomers are ring shaped
   structures constructed from five to six copies of protomers. These
   associate via non-covalent bonding to enclose the viral nucleic acid,
   though generally less intimately than helical capsids, and may involve
   one or more protomers.

   Icosahedral architecture was employed by R. Buckminster-Fuller in his
   geodesic dome, and is the most efficient way of creating an enclosed
   robust structure from multiple copies of a single protein. The number
   of proteins required to form a spherical virus capsid is denoted by the
   T-number, where 60×t proteins are necessary. In the case of the
   hepatitis B virus the T-number is 4, therefore 240 proteins assemble to
   form the capsid.
   Image Enveloped viruses
   Diagram of enveloped HIV
   Diagram of enveloped HIV
   In addition to a protein capsid many viruses are able to envelope
   themselves in a modified form of one of the cell membranes - the outer
   membrane surrounding an infected host cell, or from internal membranes
   such as nuclear membrane or endoplasmic reticulum - thus gaining an
   outer lipid bilayer known as a viral envelope. This membrane is studded
   with proteins coded for by the viral genome and host genome; however
   the lipid membrane itself and any carbohydrates present are entirely
   host-coded. The Influenza virus and HIV use this strategy.

   The viral envelope can give a virion a few distinct advantages over
   other capsid-only virions, such as protection from enzymes and certain
   chemicals. The proteins in it can include glycoproteins functioning as
   receptor molecules, allowing host cells to recognise and bind these
   virions, resulting in the possible uptake of the virion into the cell.
   Most enveloped viruses are dependent upon the envelope for infectivity.
   Image Complex viruses
   Diagram of a bacteriophage
   Diagram of a bacteriophage
   These viruses possess a capsid which is neither purely helical, nor
   purely icosahedral, and which may possess extra structures such as
   protein tails or a complex outer wall. Some bacteriophages have a
   complex structure consisting of an icosahedral head bound to a helical
   tail, the latter of which may have a hexagonal base plate with many
   protruding protein tail fibres.

   The Poxviruses are large, complex viruses which have an unusual
   morphology. The viral genome is associated with proteins within a
   central disk structure known as a nucleoid. The nucleoid is surrounded
   by a membrane and two lateral bodies of unknown function. The virus has
   an outer envelope with a thick layer of protein studded over its
   surface. The whole particle is slightly pleiomorphic, ranging from
   ovoid to brick shape.
   The range of sizes shown by viruses, relative to those of other
   organisms and biomolecules.
   The range of sizes shown by viruses, relative to those of other
   organisms and biomolecules.

Size

   To put viral size into perspective, a medium sized virion next to a
   flea is roughly equivalent to a human next to a mountain twice the size
   of Mount Everest. Some filoviruses have a total length of up to
   1400 nm, however their capsid diameters are only about 80 nm. The
   majority of viruses which have been studied have a capsid diameter
   between 10 and 300 nanometres. While most viruses are unable to be seen
   with a light microscope, some are as large or larger than the smallest
   bacteria and can be seen under high optical magnification. More
   commonly, both scanning and transmission electron microscopes are used
   to visualise virus particles.

   A notable exception to the normal viral size range is the recently
   discovered mimivirus, with a diameter of 750 nm which is larger than a
   Mycoplasma bacterium. They also hold the record for the largest viral
   genome size, possessing about 1000 genes (some bacteria only possess
   400) on a genome approximately 1.2 megabases in length. Their large
   genome also contains many genes which are conserved in both prokaryotic
   and eukaryotic genes. The discovery of the virus has led many
   scientists to reconsider the controversial boundary between living
   organisms and viruses, which are currently considered as mere mobile
   genetic elements.

Genetic material

   An electron micrograph of multiple polyomavirus virions
   An electron micrograph of multiple polyomavirus virions

   Both DNA and RNA are found in viral species, but generally a species
   will not contain both. One exception is the human cytomegalovirus,
   which contains both a DNA core and several mRNA segments. The nucleic
   acid can be either single- or double-stranded, depending on the
   species. Therefore viruses as a group contain all four possible types
   of nucleic acids: double-stranded DNA, single-stranded DNA,
   double-stranded RNA and single-stranded RNA. Animal virus species have
   been observed to possess all combinations, whereas plant viruses tend
   to have single-stranded RNA. Bacteriophages tend to have
   double-stranded DNA. Also, the nucleic acids can be either linear or a
   closed loop.

   Genome size in terms of the weight of nucleotides varies quite
   substantially between species. The smallest genomes code for only four
   proteins and weigh about 10^6 daltons, while the largest weigh about
   10^8 daltons and code for over one hundred proteins. Some virus species
   possess abnormal nucleotides, such as hydroxymethylcytosine instead of
   cytosine, as a normal part of their genome.

   For viruses with RNA as their nucleic acid, the strands are said to be
   either positive-sense (also called plus-strand) or negative-sense (also
   called minus-strand) depending on whether it is complementary to viral
   mRNA. Positive-sense viral RNA is identical to viral mRNA and thus can
   be immediately translated by the host cell. Negative-sense viral RNA is
   complementary to mRNA and thus must be converted to positive-sense RNA
   by an RNA polymerase before translation.

   All double-stranded RNA genomes and some single-stranded RNA genomes
   are said to be segmented, or divided into separate parts. Each segment
   may code for one protein, and they are usually found together in one
   capsid. Not all segments are required to be in the same virion for the
   overall virus to be infectious, as can be seen in the brome mosaic
   virus.

Replication

   Viral populations do not grow through cell division, because they are
   acellular; instead, they use the machinery and metabolism of a host
   cell to produce multiple copies of themselves. They may have a lytic or
   a lysogenic cycle, with some viruses capable of carrying out both. A
   virus can still cause degenerative effects within a cell without
   causing its death; collectively these are termed cytopathic effects.
   Released virions can be passed between hosts through either direct
   contact, often via body fluids, or through a vector. In aqueous
   environments, viruses float free in the water.

   In the lytic cycle, characteristic of virulent phages such as the T4
   phage, host cells will be induced by the virus to begin manufacturing
   the proteins necessary for virus reproduction. As well as proteins, the
   virus must also direct the replication of new genomes, the technique
   used for this varies greatly between virus species but depends heavily
   on the genome type. The final viral product is assembled spontaneously,
   though it may be aided by molecular chaperones. After the genome has
   been replicated and the new capsid assembled, the virus causes the cell
   to be broken open (lysed) to release the virus particles. Some viruses
   do not lyse the cell but instead exit the cell via the cell membrane in
   a process known as exocytosis, taking a small portion of the membrane
   with them as a viral envelope. As soon as the cell is destroyed the
   viruses have to find a new host.

   In contrast, the lysogenic cycle does not result in immediate lysing of
   the host cell, instead the viral genome integrates into the host DNA
   and replicates along with it. The virus remains dormant but after the
   host cell has replicated several times, or if environmental conditions
   permit it, the virus will become active and enter the lytic phase. The
   lysogenic cycle allows the host cell to continue to survive and
   reproduce, and the virus is passed on to all of the cell’s offspring.
   A falsely coloured electron micrograph of multiple bacteriophages
   A falsely coloured electron micrograph of multiple bacteriophages

   Bacteriophages infect specific bacteria by binding to surface receptor
   molecules and then enter the cell. Within a short amount of time,
   sometimes just minutes, bacterial polymerase starts translating viral
   mRNA into protein. These proteins go on to become either new virions
   within the cell, helper proteins which help assembly of new virions, or
   proteins involved in cell lysis. Viral enzymes aid in the breakdown of
   the cell membrane, and in the case of the T4 phage, in just over twenty
   minutes after injection over three hundred phages will be released.

   Animal DNA viruses, such as herpesviruses, enter the host via
   endocytosis, the process by which cells take in material from the
   external environment. Frequently after a chance collision with an
   appropriate surface receptor on a cell, the virus penetrates the cell,
   the viral genome is released from the capsid and host polymerases begin
   transcribing viral mRNA. New virions are assembled and released either
   by cell lysis or by budding off the cell membrane.

   Animal RNA viruses can be placed into about four different groups
   depending on their mode of replication. The polarity of the RNA largely
   determines the replicative mechanism, as well as whether the genetic
   material is single-stranded or double-stranded. Some RNA viruses are
   actually DNA based but use an RNA-intermediate to replicate. RNA
   viruses are heavily dependent upon virally encoded RNA replicase to
   create copies of their genomes.

   Reverse transcribing viruses are viruses that replicate using reverse
   transcription, which is the formation of DNA from an RNA template.
   Those viruses containing RNA genomes use a DNA intermediate to
   replicate, whereas those containing DNA genomes use an RNA intermediate
   during genome replication. Both types of reverse transcribing viruses
   use the reverse transcriptase enzyme to carry out the nucleic acid
   conversion.

Lifeform debate

   Multiple rotavirus virions
   Multiple rotavirus virions

   Argument continues over whether viruses are truly alive. According to
   the United States Code, they are considered microorganisms in the sense
   of biological weaponry and malicious use. Scientists, however, are
   divided. They have no trouble classifying a horse as living, but things
   become complicated as they look at simple viruses, viroids and prions.
   Viruses resemble other organisms in that they possess nucleic acid, and
   can respond - in infected cells - to their environment in a limited
   fashion. They can also reproduce by creating multiple copies of
   themselves through simple self-assembly.

   The concept of a virus as an organism challenges the way we define
   life: viruses do not respire, they do not display irritability, they do
   not move or grow; however, they do reproduce, and may adapt to new
   hosts by changing their genomes. By older, more zoologically and
   botanically biased criteria, then, viruses are not living. However,
   this sort of argument results from a top down sort of definition, which
   has been modified over years to take account of smaller and smaller
   things (with fewer and fewer legs, or leaves), until it has met the
   ultimate molechisms or organules - that is to say, viruses - and has
   proved inadequate. If one defines life from the bottom up - that is,
   from the simplest forms capable of displaying the most essential
   attributes of a living thing - one very quickly realises that the only
   real criterion for life is simply the ability of an organism to
   replicate, and that only systems that contain nucleic acids - in the
   natural world, at least - are capable of this phenomenon. This sort of
   reasoning has led to a new definition of organisms: "An organism is the
   unit element of a continuous lineage with an individual evolutionary
   history." The key words here are UNIT ELEMENT, and INDIVIDUAL: the
   thing that you see, now, as an organism, is merely the current slice in
   a continuous lineage; the individual evolutionary history denotes the
   independence of the organism over time. Thus, mitochondria and
   chloroplasts and nuclei and chromosomes are not organisms, in that
   together they constitute a continuous lineage, but separately have no
   possibility of survival, despite their independence before they entered
   initially symbiotic, and then dependent associations. They are also
   absent from the fossil record, making classical phylogenetic
   relationships difficult to determine: however, many workers have
   successfully inferred evolutionary histories of many millennia form
   comparing nucleotide sequences of related viruses.

   An argument can be made that all accepted forms of life use cell
   division to reproduce, whereas all viruses spontaneously assemble
   within cells. The comparison is drawn between viral self-assembly and
   the autonomous growth of non-living crystals. Virus self-assembly
   within host cells also has implications for the study of the origin of
   life, as it lends credence to the hypothesis that life could have
   started as self-assembling organic molecules.

   If viruses are considered alive, then the criteria specifying life will
   have been permanently changed, leading scientists to question what the
   basic prerequisite of life is. If they are considered living then the
   prospect of creating artificial life is enhanced, or at least the
   standards required to call something artificially alive are reduced. If
   viruses were said to be alive, the question could follow of whether
   other even smaller infectious particles, such as viroids and prions,
   would next be considered forms of life.

Viruses and disease

   Examples of common human diseases caused by viruses include the common
   cold, the flu, chickenpox and cold sores. Many serious diseases such as
   Ebola, AIDS, avian flu and SARS are also caused by viruses. The
   relative ability of viruses to cause disease is described in terms of
   virulence. Other diseases are under investigation as to whether they
   too have a virus as the causative agent, such as the possible
   connection between Human Herpesvirus Six (HHV6) and neurological
   diseases such as multiple sclerosis and chronic fatigue syndrome.
   Recently it was also shown that cervical cancer is partially caused by
   papillomavirus, representing evidence in humans of a link existing
   between cancer and an infective agent. There is current controversy
   over whether the borna virus, previously thought of as causing
   neurological disease in horses, could be responsible for psychiatric
   illness in humans.

   Viruses have many different mechanisms by which they produce disease in
   an organism, which largely depends on the species. Mechanisms at the
   cellular level primarily include cell lysis, the breaking open and
   subsequent death of the cell. In multicellular organisms, if enough
   cells die the whole organism will start to suffer the effects. Although
   many viruses result in the disruption of healthy homeostasis, resulting
   in disease, they may also exist relatively harmlessly within an
   organism. An example would include the ability of the herpes simplex
   virus, which cause coldsores, to remain in a dormant state within the
   human body.

Epidemics

   The Ebola virus
   The Ebola virus
   The Marburg virus
   The Marburg virus

   A number of highly lethal viral pathogens are members of the
   Filoviridae. Filoviruses are filament-like viruses that cause viral
   hemorrhagic fever, and include the Ebola and Marburg viruses. The
   Marburg virus attracted widespread press attention in April 2005 for an
   outbreak in Angola. Beginning in October 2004 and continuing into 2005,
   the outbreak was the world's worst epidemic of any kind of viral
   hemorrhagic fever.

   Native American populations were devastated by contagious diseases,
   particularly smallpox, brought to the Americas by European colonists.
   It is unclear how many Native Americans were killed by foreign diseases
   after the arrival of Columbus in the Americas, but the numbers have
   been estimated to be close to 70% of the indigenous population. The
   damage done by this disease may have significantly aided European
   attempts to displace or conquer the native population. Viruses also
   cause some of the most dangerous diseases ever known to man, such as
   smallpox and AIDS.

Detection, purification and diagnosis

   In the laboratory, several techniques for growing and detecting viruses
   exist. Purification of viral particles can be achieved using
   differential centrifugation, isopycnic centrifugation, precipitation
   with ammonium sulfate or ethylene glycol, and removal of cell
   components from a homogenised cell mixture using organic solvents or
   enzymes to leave the virus particles in solution.

   Assays to detect and quantify viruses include:
   A viral plaque assay
   A viral plaque assay
     * Hemagglutination assays, which quantitatively measure how many
       virus particles are in a solution of red blood cells by the amount
       of agglutination the viruses cause between them. This occurs as
       many viruses are able to bind to the surface of one or more red
       blood cells.
     * Direct counts using an electron microscope. A dilute mixture of
       virus particles and beads of known size are sprayed onto a special
       sheet and examined under high magnification. The virions are
       counted and the number extrapolated to estimate the number of
       virions in the undiluted mixture.
     * Plaque assays involve growing a thin layer of host cells onto a
       culture dish and adding a dilute mixture of virions onto it. The
       virions will infect and kill the cells they land on, producing
       holes in the cell layer known as plaques. The number of plaques can
       be counted and the number of virions estimated from it.

   Detection and subsequent isolation of new viruses from patients is a
   specialised laboratory subject. Normally it requires the use of large
   facilities, expensive equipment, and trained specialists such as
   technicians, molecular biologists, and virologists. Often, this effort
   is undertaken by state and national governments and shared
   internationally through organizations like the World Health
   Organization.

Prevention and treatment

   Because viruses use the machinery of a host cell to reproduce and also
   reside within them, they are difficult to eliminate without killing the
   host cell. The most effective medical approaches to viral diseases so
   far are vaccinations to provide resistance to infection, and drugs
   which treat the symptoms of viral infections. Patients often ask for,
   and physicians often prescribe, antibiotics. These are useless against
   viruses, and their misuse against viral infections is one of the causes
   of antibiotic resistance in bacteria. However, in life-threatening
   situations the prudent course of action is to begin a course of
   antibiotic treatment while waiting for test results to determine
   whether the patient's symptoms are caused by a virus or a bacterial
   infection.

Potential uses in therapy

   Virotherapy uses viruses as vectors to treat various diseases, as they
   can specifically target cells and DNA. It shows promising use in the
   treatment of cancer and in gene therapy.

Applications

   The polio virus
   The polio virus

Life sciences

   Viruses are important to the study of molecular and cellular biology as
   they provide simple systems that can be used to manipulate and
   investigate the functions of cells. The study and use of viruses have
   provided valuable information about many aspects of cell biology. For
   example, viruses have simplified the study of genetics and helped human
   understanding of the basic mechanisms of molecular genetics, such as
   DNA replication, transcription, RNA processing, translation, protein
   transport, and immunology.

   Geneticists regularly use viruses as vectors to introduce genes into
   cells that they are studying. This is useful for making the cell
   produce a foreign substance, or to study the effect of introducing a
   new gene into the genome. In similar fashion, virotherapy uses viruses
   as vectors to treat various diseases, as they can specifically target
   cells and DNA. It shows promising use in the treatment of cancer and in
   gene therapy.

Materials science and nanotechnology

   Current trends in nanotechnology promise to make much more versatile
   use of viruses. From the viewpoint of a materials scientist, viruses
   can be regarded as organic nanoparticles. Their surface carries
   specific tools designed to cross the barriers of their host cells. The
   size and shape of viruses, and the number and nature of the functional
   groups on their surface, is precisely defined. As such, viruses are
   commonly used in materials science as scaffolds for covalently linked
   surface modifications. A particular quality of viruses is that they can
   be tailored by directed evolution. The powerful techniques developed by
   life sciences are becoming the basis of engineering approaches towards
   nanomaterials, opening a wide range of applications far beyond biology
   and medicine.

   In April 2006 scientists at the Massachusetts Institute of Technology
   (MIT) created nanoscale metallic wires using a genetically-modified
   virus. The MIT team was able to use the virus to create a working
   battery with an energy density up to three times more than current
   materials. The potential exists for this technology to be used in
   liquid crystals, solar cells, fuel cells, and other electronics in the
   future.
   The reconstructed 1918 influenza virus
   The reconstructed 1918 influenza virus

Weapons

   The ability of viruses to cause devastating epidemics in human
   societies has led to the concern that viruses could be weaponized for
   biological warfare. Further concern was raised by the successful
   recreation of the infamous 1918 influenza virus in a laboratory. The
   smallpox virus devastated numerous societies throughout history before
   its eradication. It currently exists in several secure laboratories in
   the world, and fears that it may be used as a weapon are not totally
   unfounded. The modern global human population has almost no established
   resistance to smallpox; if it were to be released, a massive loss of
   life could be sustained before the virus is brought under control.

Etymology

   The word is from the Latin virus referring to poison and other noxious
   things, first used in English in 1392. Virulent, from Latin virulentus
   "poisonous" dates to 1400. A meaning of "agent that causes infectious
   disease" is first recorded in 1728, before the discovery of viruses by
   the Russian- Ukrainian biologist Dmitry Ivanovsky in 1892. The
   adjective viral dates to 1948. Today, virus is used to describe the
   biological viruses discussed above and also as a metaphor for other
   parasitically-reproducing things, such as memes or computer viruses
   (since 1972). The neologism virion or viron is used to refer to a
   single infective viral particle.

   The Latin word is from a Proto-Indo-European root *weis- "to melt away,
   to flow," used of foul or malodorous fluids. It is a cognate of
   Sanskrit viṣh "poison", Avestan viš- "poison", Greek ios "poison", Old
   Church Slavonic višnja "cherry", Old Irish fi "poison", Welsh gwy
   "fluid"; Latin viscum (see viscous) "sticky substance" is also from the
   same root.

   The English plural form of virus is viruses. No reputable dictionary
   gives any other form, including such "reconstructed" Latin plural forms
   as viri (which actually means men), and no plural form appears in the
   Latin corpus (See plural of virus). The word does not have a
   traditional Latin plural because its original sense, poison is a mass
   noun like the English word furniture, and, as pointed out above,
   English use of virus to denote the agent of a disease predates the
   discovery that these agents are microscopic parasites and thus in
   principle countable.

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