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{{short description|Non-technical introduction to viruses}}
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{{Use British English|date=March 2020}}
{{Use dmy dates|date=March 2014}}
{{featured article}}
{{introductory article|Virus}}
{{About|the type of pathogen|the type of malware|Computer virus|other uses|Virus (disambiguation)}}
[[File:Coronavirus. SARS-CoV-2.png|thumb|right|Illustration of a [[Severe acute respiratory syndrome coronavirus 2|SARS-CoV-2]] virion]]
A '''virus''' is a tiny [[infectious agent]] that [[reproduction|reproduces]] inside the [[Cell (biology)|cells]] of living [[Host (biology)|hosts]]. When infected, the host cell is forced to rapidly produce thousands of identical copies of the original virus. Unlike most [[Organism|living things]], viruses do not have cells that divide; new viruses assemble in the infected host cell. But unlike simpler infectious agents like [[prion]]s, they contain [[Introduction to genetics|genes]], which allow them to [[Mutation|mutate]] and evolve. Over 4,800 [[List of virus species|species of viruses]] have been [[List of virus taxa|described in detail]]<ref name="pmid29754305">{{cite journal |vauthors=King AM, Lefkowitz EJ, Mushegian AR, Adams MJ, Dutilh BE, Gorbalenya AE, Harrach B, Harrison RL, Junglen S, Knowles NJ, Kropinski AM, Krupovic M, Kuhn JH, Nibert ML, Rubino L, Sabanadzovic S, Sanfaçon H, Siddell SG, Simmonds P, Varsani A, Zerbini FM, Davison AJ |s2cid=21670772 |title=Changes to taxonomy and the International Code of Virus Classification and Nomenclature ratified by the International Committee Taxonomy of Viruses (2018) |journal=Archives of Virology |volume=163 |issue=9 |date=September 2018 |page=2601 <!-- CITATION BOT DO NOT CHANGE THIS ... citation is specific to this page --> |pmid=29754305 |doi=10.1007/s00705-018-3847-1 |url=https://hal-pasteur.archives-ouvertes.fr/pasteur-01977332/file/King2018_Article_ChangesToTaxonomyAndTheInterna.pdf}}</ref> out of the millions in the environment. Their origin is unclear: some may have [[evolution|evolved]] from [[plasmid]]s—pieces of DNA that can move between cells—while others may have evolved from [[bacteria]].
Viruses are made of either two or three parts. All include [[gene]]s. These genes contain the encoded biological information of the virus and are built from either [[DNA]] or [[RNA]]. All viruses are also covered with a [[protein]] coat to protect the genes. Some viruses may also have an [[viral envelope|envelope]] of [[Lipid|fat-like substance]] that covers the protein coat, and makes them vulnerable to soap. A virus with this "viral envelope" uses it—along with specific [[Cell surface receptor|receptors]]—to enter a new host cell. Viruses vary in shape from the simple [[tobacco mosaic virus|helical]] and [[icosahedron|icosahedral]] to more [[bacteriophage|complex]] structures. Viruses range in size from 20 to 300 [[nanometre]]s; it would take 33,000 to 500,000 of them, laid end to end, to stretch to {{convert|1|cm|1}}.
Viruses spread in many ways. Although many are very specific about which host species or [[Tissue (biology)|tissue]] they attack, each [[species]] of virus relies on a particular method to copy itself. [[Plant pathology#Viruses, viroids and virus-like organisms|Plant viruses]] are often spread from plant to plant by insects and other [[organism]]s, known as ''[[Vector (epidemiology)|vectors]]''. Some [[Virus#Role in human disease|viruses of humans]] and other animals are spread by exposure to infected bodily fluids. Viruses such as [[influenza]] are spread through the air by droplets of moisture when people cough or sneeze. Viruses such as [[norovirus]] are transmitted by the [[fecal–oral route|faecal–oral route]], which involves the contamination of hands, food and water. [[Rotavirus]] is often spread by direct contact with infected children. The human immunodeficiency virus, [[HIV]], is transmitted by bodily fluids transferred during sex. Others, such as the [[dengue virus]], are spread by [[Hematophagy|blood-sucking insects]].
Viruses, especially those made of RNA, can [[Mutation|mutate]] rapidly to give rise to new types. Hosts may have little [[Immunity (medical)|protection]] against such new forms. Influenza virus, for example, changes often, so a new [[vaccine]] is needed each year. Major changes can cause [[pandemic]]s, as in the [[2009 flu pandemic|2009 swine influenza]] that spread to most countries. Often, these mutations take place when the virus has first infected other animal hosts. Some examples of such [[Zoonosis|"zoonotic" diseases]] include [[coronavirus]] in bats, and influenza in pigs and birds, before those viruses were [[Cross-species transmission|transferred to humans]].
Viral infections can cause disease in humans, animals and plants. In healthy humans and animals, infections are usually eliminated by the [[immune system]], which can provide lifetime [[Immunity (medical)|immunity]] to the host for that virus. [[Antibiotic]]s, which work against bacteria, have no impact, but [[antiviral drug]]s can treat life-threatening infections. Those vaccines that produce lifelong immunity can prevent some infections.
{{Virus glossary}}
==
{{Main|History of virology}}
[[File:HIV-budding-Color cropped.jpg|left|upright|thumbnail|[[Scanning electron microscope|Scanning electron micrograph]] of HIV-1 viruses, coloured green, budding from a [[lymphocyte]]]]
In 1884, French [[microbiologist]] [[Charles Chamberland]] invented the [[Chamberland filter]] (or Chamberland–Pasteur filter), that contains pores smaller than [[bacteria]]. He could then pass a solution containing bacteria through the filter, and completely remove them. In the early 1890s, Russian [[biologist]] [[Dmitri Ivanovsky]] used this method to study what became known as the [[tobacco mosaic virus]]. His experiments showed that extracts from the crushed leaves of infected tobacco plants remain infectious after filtration.<ref>{{harvnb|Shors|2017|p=6}}</ref>
At the same time, several other scientists showed that, although these agents (later called viruses) were different from bacteria and about one hundred times smaller, they could still cause disease. In 1899, Dutch microbiologist [[Martinus Beijerinck]] observed that the agent only multiplied when in [[cell division|dividing cells]]. He called it a "contagious living fluid" ({{langx|la|text= [[contagium vivum fluidum]]}})—or a "soluble living germ" because he could not find any germ-like particles.{{sfn|Howley|Knipe|Enquist|2023|p=4}} In the early 20th century, English [[bacteriologist]] [[Frederick Twort]] discovered viruses that infect bacteria,<ref>{{harvnb|Shors|2017|p=827}}</ref> and French-Canadian microbiologist [[Félix d'Herelle]] described viruses that, when added to bacteria growing on [[agar]], would lead to the formation of whole areas of dead bacteria. Counting these dead areas allowed him to calculate the number of viruses in the suspension.<ref>{{cite journal | vauthors = D'Herelle F | title = On an invisible microbe antagonistic toward dysenteric bacilli: brief note by Mr. F. D'Herelle, presented by Mr. Roux. 1917 | journal = Research in Microbiology | volume = 158 | issue = 7 | pages = 553–554 | year = 2007 | pmid = 17855060 | doi = 10.1016/j.resmic.2007.07.005 | doi-access = free }}</ref>
The invention of the [[electron microscope]] in 1931 brought the first images of viruses.<ref>From ''Nobel Lectures, Physics 1981–1990'', (1993) Editor-in-Charge Tore Frängsmyr, Editor Gösta Ekspång, World Scientific Publishing Co., Singapore</ref> In 1935, American [[biochemist]] and [[virologist]] [[Wendell Meredith Stanley]] examined the tobacco mosaic virus (TMV) and found it to be mainly made from [[protein]].<ref>{{cite journal | vauthors = Stanley WM, Loring HS | year = 1936 | title = The isolation of crystalline tobacco mosaic virus protein from diseased tomato plants | journal = Science | volume = 83 | issue = 2143| page = 85 | pmid = 17756690 | doi = 10.1126/science.83.2143.85 |bibcode = 1936Sci....83...85S }}</ref> A short time later, this virus was shown to be made from protein and [[RNA]].<ref>{{cite journal | doi = 10.1126/science.89.2311.345 | vauthors = Stanley WM, Lauffer MA | year = 1939 | title = Disintegration of tobacco mosaic virus in urea solutions |journal = Science | volume = 89 | issue = 2311| pages = 345–347 | pmid = 17788438 |bibcode = 1939Sci....89..345S }}</ref> [[Rosalind Franklin]] developed [[X-ray crystallography|X-ray crystallographic pictures]] and determined the full structure of TMV in 1955.<ref name="pmid18702397">{{cite journal | vauthors = Creager AN, Morgan GJ | title = After the double helix: Rosalind Franklin's research on Tobacco mosaic virus | journal = Isis; an International Review Devoted to the History of Science and Its Cultural Influences | volume = 99 | issue = 2 | pages = 239–272 | date = June 2008 | pmid = 18702397 | doi = 10.1086/588626 | s2cid = 25741967 }}</ref> Franklin confirmed that viral proteins formed a spiral hollow tube, wrapped by RNA, and also showed that viral RNA was a single strand, not a double helix like DNA.<ref name="Johnson">{{cite journal |last1=Johnson |first1=Ben |title=Rosalind Franklin's contributions to virology |journal=Nature Portfolio Microbiology Community |date=25 July 2017 |url=https://microbiologycommunity.nature.com/posts/18900-rosalind-franklin-s-contributions-to-virology |access-date=7 January 2022 |language=en}}</ref>
A problem for early scientists was that they did not know how to grow viruses without using live animals. The breakthrough came in 1931, when American [[pathologist]]s [[Ernest William Goodpasture]] and [[Alice Miles Woodruff]] grew [[influenza]], and several other viruses, in fertilised chickens' eggs.<ref name="pmid17810781">{{cite journal | vauthors = Goodpasture EW, Woodruff AM, Buddingh GJ | title = The Cultivation Of Vaccine and other Viruses In The Chorioallantoic Membrane of Chick Embryos | journal = Science | volume = 74 | issue = 1919 | pages = 371–372 | date = October 1931 | pmid = 17810781 | doi = 10.1126/science.74.1919.371 | bibcode = 1931Sci....74..371G}}</ref> Some viruses could not be grown in chickens' eggs. This problem was solved in 1949, when [[John Franklin Enders]], [[Thomas Huckle Weller]], and [[Frederick Chapman Robbins]] grew [[polio virus]] in cultures of living animal cells.<ref name="pmid15470207">{{cite journal | vauthors = Rosen FS | title = Isolation of poliovirus – John Enders and the Nobel Prize | journal = N. Engl. J. Med. | volume = 351 | issue = 15 | pages = 1481–1483 | date = October 2004 | pmid = 15470207 | doi = 10.1056/NEJMp048202 }}</ref> Over 4,800 species of viruses have been [[List of virus taxa|described in detail]].<ref name="pmid29754305"/>
==
{{Further|Virus#Origins}}
Viruses co-exist with life wherever it occurs. They have probably existed since living cells first evolved. Their origin remains unclear because they do not [[fossil]]ize, so [[Molecular biology|molecular techniques]] have been the best way to [[hypothesis]]e about how they arose. These techniques rely on the availability of ancient viral DNA or RNA, but most viruses that have been preserved and stored in laboratories are less than 90 years old.<ref>{{harvnb|Shors|2017|p=16}}</ref><ref>{{harvnb|Collier|Balows|Sussman|1998|pp=18–19}}</ref> Molecular methods have only been successful in tracing the ancestry of viruses that evolved in the 20th century.<ref name="pmid15476878">{{cite journal | vauthors = Liu Y, Nickle DC, Shriner D, Jensen MA, Learn GH, Mittler JE, Mullins JI | title = Molecular clock-like evolution of human immunodeficiency virus type 1 | journal = Virology | volume = 329 | issue = 1 | pages = 101–108 | date = November 2004 | pmid = 15476878 | doi = 10.1016/j.virol.2004.08.014| doi-access = free }}</ref> New groups of viruses might have repeatedly emerged at all stages of the evolution of life.<ref name=NRM_Krupovic2019>{{cite journal |vauthors= Krupovic M, Dooja W, Koonin EV |s2cid=169035711 |title=Origin of viruses: primordial replicators recruiting capsids from hosts. |journal=Nature Reviews Microbiology |volume=17 |issue=7 |pages=449–458 |date=2019 |doi=10.1038/s41579-019-0205-6 |pmid=31142823|url=https://hal-pasteur.archives-ouvertes.fr/pasteur-02557191/file/Krupovic_NRMICRO-19-022_MS_v3_clean.pdf }}</ref> There are three major [[Scientific theory|theories]] about the origins of viruses:<ref name=NRM_Krupovic2019 /><ref>{{harvnb|Collier|Balows|Sussman|1998|pp=11–21}}</ref>
; Regressive theory : Viruses may have once been small cells that [[parasitism|parasitised]] larger cells. Eventually, the genes they no longer needed for a parasitic way of life were lost. The bacteria ''[[Rickettsia]]'' and ''[[Chlamydia (bacterium)|Chlamydia]]'' are living cells that, like viruses, can reproduce only inside host cells. This lends credence to this theory, as their dependence on being parasites may have led to the loss of the genes that once allowed them to live on their own.<ref>{{harvnb|Collier|Balows|Sussman|1998|p=11}}</ref>
; Cellular origin theory : Some viruses may have evolved from bits of DNA or RNA that "escaped" from the genes of a larger organism. The escaped DNA could have come from [[plasmid]]s—pieces of DNA that can move between cells—while others may have evolved from bacteria.<ref>{{harvnb|Collier|Balows|Sussman|1998|pp=11–12}}</ref>
; Coevolution theory : Viruses may have evolved from complex molecules of protein and DNA at the same time as cells first appeared on earth, and would have depended on cellular life for many millions of years.<ref name =Wessner>{{cite journal | vauthors = Wessner DR | year = 2010 | title = The Origins of Viruses | journal = Nature Education | volume = 3 | issue = 9| page = 37 }}</ref>
There are problems with all of these theories. The regressive hypothesis does not explain why even the smallest of cellular parasites do not resemble viruses in any way. The escape or the cellular origin hypothesis does not explain the presence of unique structures in viruses that do not appear in cells. The coevolution, or "virus-first" hypothesis, conflicts with the definition of viruses, because viruses depend on host cells.<ref name =Wessner /><ref name=Nasir>{{cite journal|title=Viral evolution: Primordial cellular origins and late adaptation to parasitism | vauthors= Nasir A, Kim KM, Caetano-Anollés G | year = 2012 | pmid = 23550145 | doi=10.4161/mge.22797 | journal=Mobile Genetic Elements | volume = 2 | issue =5 | pages=247–252| pmc= 3575434 }}</ref> Also, viruses are recognised as ancient, and to have origins that pre-date the divergence of life into the [[Three-___domain system|three domains]].<ref name="Mahy Gen 28" /> This discovery has led modern virologists to reconsider and re-evaluate these three classical hypotheses.<ref name=NRM_Krupovic2019 /><ref name="Mahy Gen 28">{{cite book | vauthors = Mahy WJ, Van Regenmortel MH |title=Desk Encyclopedia of General Virology |publisher=Academic Press |___location=Oxford |year=2009 |page=28 |isbn=978-0-12-375146-1}}</ref>
== Structure ==
A virus particle, also called a [[virion]], consists of genes made from DNA or RNA which are surrounded by a protective coat of protein called a [[capsid]].{{sfn|Howley|Knipe|Enquist|2023|pp=50-52}}{{harvnb|Collier|Balows|Sussman|1998|pp=33–55}} The capsid is made of many smaller, identical protein molecules called [[capsomer]]s. The arrangement of the capsomers can either be [[truncated icosahedron|icosahedral]] (20-faced), [[helix|helical]], or more complex. There is an inner shell around the DNA or RNA called the [[nucleocapsid]], made out of proteins. Some viruses are surrounded by a bubble of [[lipid]] (fat) called an [[viral envelope|envelope]], which makes them vulnerable to soap and alcohol.<ref name="pmid11759024">{{cite journal |vauthors=Rotter ML |title=Arguments for alcoholic hand disinfection |journal=The Journal of Hospital Infection |volume=48 |issue=Suppl A |pages=S4–S8 |date=August 2001 |pmid=11759024 |doi=10.1016/s0195-6701(01)90004-0 }}</ref>
===
[[File:Virus size.png|right|thumb|Virions of some of the most common human viruses with their relative size. The nucleic acids are not to scale.]]
Viruses are among the smallest infectious agents, and are too small to be seen by [[Optical microscope|light microscopy]]; most of them can only be seen by [[electron microscopy]]. Their sizes range from 20 to 300 [[nanometre]]s; it would take 30,000 to 500,000 of them, laid end to end, to stretch to one centimetre (0.4 in).<ref name="Topley-and-Wilson33-55">{{harvnb|Collier|Balows|Sussman|1998|pp=33–55}}</ref> In comparison, bacteria are typically around 1000 nanometres (1 [[micrometre|micrometer]]) in diameter, and host cells of higher organisms are typically a few tens of micrometers. Some viruses, such as [[megavirus]]es and [[pandoravirus]]es, are relatively large viruses. At around 1000 nanometres, these viruses, which infect [[amoeba]]e, were discovered in 2003 and 2013.<ref>{{cite journal |vauthors=Abergel C, Legendre M, Claverie JM |title=The rapidly expanding universe of giant viruses: Mimivirus, Pandoravirus, Pithovirus and Mollivirus |journal=FEMS Microbiol. Rev. |volume=39 |issue=6 |pages=779–796 |date=November 2015 |pmid=26391910 |doi=10.1093/femsre/fuv037 |doi-access=free }}</ref><ref>{{cite journal |vauthors=Philippe N, Legendre M, Doutre G, Couté Y, Poirot O, Lescot M, Arslan D, Seltzer V, Bertaux L, Bruley C, Garin J, Claverie JM, Abergel C |s2cid=16877147 |title=Pandoraviruses: amoeba viruses with genomes up to 2.5 Mb reaching that of parasitic eukaryotes |journal=Science |volume=341 |issue=6143 |pages=281–286 |date=July 2013 |pmid=23869018 |doi=10.1126/science.1239181 |bibcode=2013Sci...341..281P |url=https://hal-cea.archives-ouvertes.fr/cea-00862677/file/phi.pdf }}</ref> They are around ten times wider (and thus a thousand times larger in volume) than [[influenza virus]]es, and the discovery of these "giant" viruses astonished scientists.<ref name="Zimmer">{{cite news | url=https://www.nytimes.com/2013/07/18/science/changing-view-on-viruses-not-so-small-after-all.html | title=Changing View on Viruses: Not So Small After All | work=The New York Times | date=18 July 2013 | access-date=20 December 2014 | vauthors = Zimmer C}}</ref>
=== Genes ===
{{details|Introduction to genetics}}
The genes of viruses are made from DNA (deoxyribonucleic acid) and, in many viruses, RNA (ribonucleic acid). The biological information contained in an organism is [[Genetic code|encoded]] in its DNA or RNA. Most organisms use DNA, but [[RNA virus|many viruses]] have RNA as their genetic material. The DNA or RNA of viruses consists of either a single strand or a double helix.<ref>{{harvnb|Shors|2017|p=81}}</ref>
Viruses can reproduce rapidly because they have relatively few genes. For example, influenza virus has only eight genes and [[rotavirus]] has eleven. In comparison, humans have 20,000–25,000. Some viral genes contain the code to make the structural proteins that form the virus particle. Other genes make non-structural proteins found only in the cells the virus infects.<ref>{{harvnb|Shors|2017|p=129}}</ref><ref>{{cite journal | doi = 10.1038/nature03001 | last1 = International Human | first1 = Genome Sequencing Consortium | year = 2004 | title = Finishing the euchromatic sequence of the human genome | journal = Nature | volume = 431 | issue = 7011| pages = 931–945 | pmid = 15496913 |bibcode = 2004Natur.431..931H | s2cid = 186242248 | doi-access = free }}</ref>
All cells, and many viruses, produce proteins that are [[enzyme]]s that drive chemical reactions. Some of these enzymes, called [[DNA polymerase]] and [[RNA polymerase]], make new copies of DNA and RNA. A virus's polymerase enzymes are often much more efficient at making DNA and RNA than the equivalent enzymes of the host cells,<ref>{{harvnb|Shors|2017|pp=129–31}}</ref> but viral RNA polymerase enzymes are error-prone, causing RNA viruses to mutate and form new strains.<ref>{{harvnb|Shors|2017|p=652}}</ref>
In some species of RNA virus, the genes are not on a continuous molecule of RNA, but are separated. The influenza virus, for example, has eight separate genes made of RNA. When two different strains of influenza virus infect the same cell, these genes can mix and produce new strains of the virus in a process called [[reassortment]].<ref>{{harvnb|Shors|2017|p=654}}</ref>
=== Protein synthesis ===
[[File:Cell with Virus.png|thumb|upright=1.3|Diagram of a typical [[eukaryotic]] cell, showing subcellular components. [[Organelle]]s: (1) [[nucleolus]] (2) [[Cell nucleus|nucleus]] (3) [[ribosome]] (4) [[vesicle (biology)|vesicle]] (5) rough [[endoplasmic reticulum]] (ER) (6) [[Golgi apparatus]] (7) [[cytoskeleton]] (8) smooth ER (9) [[Mitochondrion|mitochondria]] (10) [[vacuole]] (11) [[cytoplasm]] (12) [[lysosome]] (13) [[centriole]]s within [[centrosome]] (14) a virus shown to approximate scale]]
Proteins are essential to life. Cells produce new protein molecules from [[amino acid]] building blocks based on information coded in DNA. Each type of protein is a specialist that usually only performs one function, so if a cell needs to do something new, it must make a new protein. Viruses force the cell to make new proteins that the cell does not need, but are needed for the virus to reproduce. [[Protein biosynthesis|Protein synthesis]] consists of two major steps: [[Transcription (genetics)|transcription]] and [[Translation (biology)|translation]].<ref name="pmid25648499">{{cite journal |vauthors=de Klerk E, 't Hoen PA |title=Alternative mRNA transcription, processing, and translation: insights from RNA sequencing |journal=Trends in Genetics |volume=31 |issue=3 |pages=128–139 |date=March 2015 |pmid=25648499 |doi=10.1016/j.tig.2015.01.001 }}</ref>
Transcription is the process where information in DNA, called the [[genetic code]], is used to produce RNA copies called [[messenger RNA]] (mRNA). These migrate through the cell and carry the code to [[ribosome]]s where it is used to make proteins. This is called translation because the protein's amino acid structure is determined by the mRNA's code. Information is hence translated from the language of nucleic acids to the language of amino acids.<ref name="pmid25648499" />
Some nucleic acids of RNA viruses function directly as mRNA without further modification. For this reason, these viruses are called positive-sense RNA viruses.<ref>{{harvnb|Collier|Balows|Sussman|1998|pp=75–82}}</ref> In other RNA viruses, the RNA is a complementary copy of mRNA and these viruses rely on the cell's or their own enzyme to make mRNA. These are called [[Sense (molecular biology)|negative-sense]] RNA viruses. In viruses made from DNA, the method of mRNA production is similar to that of the cell. The species of viruses called [[retrovirus]]es behave completely differently: they have RNA, but inside the host cell a DNA copy of their RNA is made with the help of the enzyme [[reverse transcriptase]]. This DNA is then incorporated into the host's own DNA, and copied into mRNA by the cell's normal pathways.<ref>{{harvnb|Shors|2017|p=698}}</ref>
== Life-cycle ==
{{Main|Viral life cycle|Viral entry}}
[[File:HepC replication.png|thumb|Life-cycle of a typical virus (left to right); following infection of a cell by a single virus, hundreds of offspring are released.]]
When a virus infects a cell, the virus forces it to make thousands more viruses. It does this by making the cell copy the virus's DNA or RNA, making viral proteins, which all assemble to form new virus particles.<ref>{{harvnb|Shors|2017|pp=6–13}}</ref>
There are six basic, overlapping stages in the life cycle of viruses in living cells:<ref>{{harvnb|Shors
*'''Attachment''' is the binding of the virus to specific molecules on the surface of the cell. This specificity restricts the virus to a very limited type of cell. For example, the human immunodeficiency virus (HIV) infects only human [[T cell]]s, because its surface protein, [[gp120]], can only react with [[CD4]] and other molecules on the T cell's surface. Plant viruses can only attach to plant cells and cannot infect animals. This mechanism has evolved to favour those viruses that only infect cells in which they are capable of reproducing.
*'''Penetration''' follows attachment; viruses penetrate the host cell by [[endocytosis]] or by fusion with the cell.
*'''Uncoating''' happens inside the cell when the viral capsid is removed and destroyed by viral enzymes or host enzymes, thereby exposing the viral nucleic acid.
*'''Replication''' of virus particles is the stage where a cell uses viral messenger RNA in its protein synthesis systems to produce viral proteins. The RNA or DNA synthesis abilities of the cell produce the virus's DNA or RNA.
*'''Assembly''' takes place in the cell when the newly created viral proteins and nucleic acid combine to form hundreds of new virus particles.
*'''Release''' occurs when the new viruses escape or are released from the cell. Most viruses achieve this by making the cells burst, a process called [[lysis]]. Other viruses such as HIV are released more gently by a process called [[viral shedding#Via budding|budding]].
== Effects on the host cell ==
Viruses have an extensive range of structural and biochemical effects on the host cell.{{sfn | Oxford |Kellam|Collier| 2016 | p=34–36}} These are called ''[[cytopathic effect]]s''.{{sfn | Oxford |Kellam|Collier| 2016 | p=34}} Most virus infections eventually result in the death of the host cell. The causes of death include cell lysis (bursting), alterations to the cell's surface membrane and [[apoptosis]] (cell "suicide").<ref name="pmid28846635">{{cite journal |vauthors=Okamoto T, Suzuki T, Kusakabe S, Tokunaga M, Hirano J, Miyata Y, Matsuura Y |title=Regulation of Apoptosis during Flavivirus Infection |journal=Viruses |volume=9 |issue=9 |pages= 243|year= 2017 |pmid=28846635 |pmc=5618009 |doi=10.3390/v9090243|doi-access=free }}</ref> Often cell death is caused by cessation of its normal activity due to proteins produced by the virus, not all of which are components of the virus particle.<ref name="pmid18637511">{{cite book |vauthors=Alwine JC |title=Human Cytomegalovirus |chapter=Modulation of Host Cell Stress Responses by Human Cytomegalovirus |series=Current Topics in Microbiology and Immunology |volume=325|pages=263–79 |date=2008 |pmid=18637511 |doi=10.1007/978-3-540-77349-8_15 |isbn=978-3-540-77348-1 }}</ref>
Some viruses cause no apparent changes to the infected cell. Cells in which the virus is [[virus latency|latent]] (inactive) show few signs of infection and often function normally.<ref name="pmid18164651">{{cite journal | vauthors = Sinclair J | title = Human cytomegalovirus: Latency and reactivation in the myeloid lineage | journal = J. Clin. Virol. | volume = 41 | issue = 3 | pages = 180–185 | date = March 2008 | pmid = 18164651 | doi = 10.1016/j.jcv.2007.11.014 }}</ref> This causes persistent infections and the virus is often dormant for many months or years. This is often the case with [[herpes simplex|herpes viruses]].<ref name="pmid6326635">{{cite journal | vauthors = Jordan MC, Jordan GW, Stevens JG, Miller G | title = Latent herpesviruses of humans | journal = Ann. Intern. Med. | volume = 100 | issue = 6 | pages = 866–880 | date = June 1984 | pmid = 6326635 | doi = 10.7326/0003-4819-100-6-866 }}</ref><ref name="pmid12076064">{{cite journal | vauthors = Sissons JG, Bain M, Wills MR | s2cid = 24879226 | title = Latency and reactivation of human cytomegalovirus | journal = J. Infect. | volume = 44 | issue = 2 | pages = 73–77 | date = February 2002 | pmid = 12076064 | doi = 10.1053/jinf.2001.0948}}</ref>
Some viruses, such as [[Epstein–Barr virus]], often cause cells to proliferate without causing [[malignancy]];<ref name="pmid18035323">{{cite journal | vauthors = Barozzi P, Potenza L, Riva G, Vallerini D, Quadrelli C, Bosco R, Forghieri F, Torelli G, Luppi M | title = B cells and herpesviruses: a model of lymphoproliferation | journal = Autoimmun Rev | volume = 7 | issue = 2 | pages = 132–136 | date = December 2007 | pmid = 18035323 | doi = 10.1016/j.autrev.2007.02.018 | hdl = 11380/598275 | hdl-access = free }}</ref> but some other viruses, such as [[papillomavirus]], are an established cause of cancer.<ref name="pmid28798073">{{cite journal |vauthors=Graham SV |title=The human papillomavirus replication cycle, and its links to cancer progression: a comprehensive review |journal=Clinical Science |volume=131 |issue=17 |pages=2201–2221 |year= 2017 |pmid=28798073 |doi=10.1042/CS20160786 |doi-access=free }}</ref> When a cell's DNA is damaged by a virus such that the cell cannot repair itself, this often triggers apoptosis. One of the results of apoptosis is destruction of the damaged DNA by the cell itself. Some viruses have mechanisms to limit apoptosis so that the host cell does not die before progeny viruses have been produced; [[HIV]], for example, does this.<ref name="pmid10547702">{{cite journal | vauthors = Roulston A, Marcellus RC, Branton PE | title = Viruses and apoptosis | journal = Annu. Rev. Microbiol. | volume = 53 | pages = 577–628 | date = 1999 | pmid = 10547702 | doi = 10.1146/annurev.micro.53.1.577 }}</ref>
== Viruses and diseases ==
There are many ways in which viruses spread from host to host but each species of virus uses only one or two. Many viruses that infect plants are carried by [[organism]]s; such organisms are called [[Vector (epidemiology)|vectors]]. Some viruses that infect animals, including humans, are also spread by vectors, usually blood-sucking insects, but direct transmission is more common. Some virus infections, such as [[norovirus]] and [[rotavirus]], are spread by contaminated food and water, by hands and communal [[fomites|objects]], and by intimate contact with another infected person, while others like [[SARS-CoV-2]] and influenza viruses are airborne. Viruses such as HIV, [[hepatitis B]] and [[hepatitis C]] are often transmitted by unprotected sex or contaminated [[hypodermic needle]]s. To prevent infections and epidemics, it is important to know how each different kind of virus is spread.<ref>{{harvnb|Shors|2017|p=32}}</ref>
==
Common human diseases caused by viruses include the [[common cold]], [[influenza]], [[chickenpox]] and [[cold sores]]. Serious diseases such as [[Ebola]] and [[AIDS]] are also caused by viruses.<ref>{{harvnb|Shors|2017|p=271}}</ref> Many viruses cause little or no disease and are said to be "benign". The more harmful viruses are described as [[virulence|virulent]].<ref>{{cite journal|vauthors =Berngruber TW, Froissart R, Choisy M, Gandon S|year= 2013|title = Evolution of Virulence in Emerging Epidemics|journal = PLOS Pathogens|volume= 9 |issue= 3|pages= e1003209|doi= 10.1371/journal.ppat.1003209|pmid= 23516359|pmc= 3597519|doi-access= free}}</ref>
Viruses cause different diseases depending on the types of cell that they infect.
Some viruses can cause lifelong or [[Chronic (medical)|chronic]] infections where the viruses continue to reproduce in the body despite the host's defence mechanisms.<ref>{{harvnb|Shors|2017|p=464}}</ref> This is common in hepatitis B virus and hepatitis C virus infections. People chronically infected with a virus are known as carriers. They serve as important reservoirs of the virus.<ref name="pmid31364248">{{cite journal |vauthors=Tanaka J, Akita T, Ko K, Miura Y, Satake M |title=Countermeasures against viral hepatitis B and C in Japan: An epidemiological point of view |journal=Hepatology Research |volume=49 |issue=9 |pages=990–1002 |date=September 2019 |pmid=31364248 |pmc=6852166 |doi=10.1111/hepr.13417 }}</ref><ref name="pmid32173241">{{cite journal |vauthors=Lai CC, Liu YH, Wang CY, Wang YH, Hsueh SC, Yen MY, Ko WC, Hsueh PR |title=Asymptomatic carrier state, acute respiratory disease, and pneumonia due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2): Facts and myths |journal=Journal of Microbiology, Immunology, and Infection = Wei Mian Yu Gan Ran Za Zhi |volume= 53|issue= 3|pages= 404–412|date=March 2020 |pmid=32173241 |doi=10.1016/j.jmii.2020.02.012 |pmc=7128959 }}</ref>
==== Endemic ====
If the proportion of carriers in a given population reaches a given threshold, a disease is said to be [[Endemic (epidemiology)|endemic]].{{sfn | Oxford |Kellam|Collier| 2016 | p=63}} Before the advent of vaccination, infections with viruses were common and outbreaks occurred regularly. In countries with a temperate climate, viral diseases are usually seasonal. [[Poliomyelitis]], caused by [[poliovirus]] often occurred in the summer months.<ref name="pmid29961515">{{cite journal |vauthors=Strand LK |title=The Terrible Summer of 1952 … When Polio Struck Our Family |journal=Seminars in Pediatric Neurology |volume=26 |pages=39–44 |date=July 2018 |pmid=29961515 |doi=10.1016/j.spen.2017.04.001 |s2cid=49640682 }}</ref> By contrast colds, influenza and rotavirus infections are usually a problem during the winter months.<ref name="pmid22958213">{{cite journal |vauthors=Moorthy M, Castronovo D, Abraham A, Bhattacharyya S, Gradus S, Gorski J, Naumov YN, Fefferman NH, Naumova EN |title=Deviations in influenza seasonality: odd coincidence or obscure consequence? |journal=Clinical Microbiology and Infection |volume=18 |issue=10 |pages=955–962 |date=October 2012 |pmid=22958213 |pmc=3442949 |doi=10.1111/j.1469-0691.2012.03959.x }}</ref><ref name="pmid25777068">{{cite journal |vauthors=Barril PA, Fumian TM, Prez VE, Gil PI, Martínez LC, Giordano MO, Masachessi G, Isa MB, Ferreyra LJ, Ré VE, Miagostovich M, Pavan JV, Nates SV |title=Rotavirus seasonality in urban sewage from Argentina: effect of meteorological variables on the viral load and the genetic diversity |journal=Environmental Research |volume=138 |pages=409–415 |date=April 2015 |pmid=25777068 |doi=10.1016/j.envres.2015.03.004 |bibcode=2015ER....138..409B |hdl=11336/61497 |hdl-access=free }}</ref> Other viruses, such as [[measles virus]], caused outbreaks regularly every third year.<ref name="pmid25444814">{{cite journal |vauthors=Durrheim DN, Crowcroft NS, Strebel PM |title=Measles – The epidemiology of elimination |journal=Vaccine |volume=32 |issue=51 |pages=6880–6883 |date=December 2014 |pmid=25444814 |doi=10.1016/j.vaccine.2014.10.061 |doi-access=free |hdl=1959.13/1299149 |hdl-access=free }}</ref> In developing countries, viruses that cause respiratory and enteric infections are common throughout the year. Viruses carried by insects are a common cause of diseases in these settings. [[Zika]] and [[dengue virus]]es for example are transmitted by female [[Aedes]] mosquitoes, which bite humans particularly during the mosquitoes' breeding season.<ref name="pmid32103776">{{cite journal |vauthors=Mbanzulu KM, Mboera LE, Luzolo FK, Wumba R, Misinzo G, Kimera SI |title=Mosquito-borne viral diseases in the Democratic Republic of the Congo: a review |journal=Parasites & Vectors |volume=13 |issue=1 |article-number=103 |date=February 2020 |pmid=32103776 |pmc=7045448 |doi=10.1186/s13071-020-3985-7 |doi-access=free }}</ref>
==== Pandemic and emergent ====
[[File:SIV primates.jpg|right|400px|thumb|Left to right: the [[African green monkey]], source of [[Simian immunodeficiency virus|SIV]]; the [[sooty mangabey]], source of [[HIV-2]]; and the [[Common chimpanzee|chimpanzee]], source of [[HIV-1]]]]
[[File:Orgin and evolution of SARS.jpg|thumb|Origin and evolution of (A) SARS-CoV, (B) MERS-CoV, and (C) SARS-CoV-2 in different hosts. All the viruses came from bats as coronavirus-related viruses before mutating and adapting to intermediate hosts and then to humans and causing the diseases [[SARS]], [[MERS]] and [[COVID-19]]. (<small>Adapted from Ashour et al. (2020)</small> <ref name="pmid32143502">{{cite journal |vauthors=Ashour HM, Elkhatib WF, Rahman MM, Elshabrawy HA |title=Insights into the Recent 2019 Novel Coronavirus (SARS-CoV-2) in Light of Past Human Coronavirus Outbreaks |journal=Pathogens (Basel, Switzerland) |volume=9 |issue=3 |pages= 186|date=March 2020 |pmid=32143502 |doi=10.3390/pathogens9030186 |pmc=7157630 |doi-access=free }}</ref>)]]
Although viral [[pandemic]]s are rare events, HIV—which evolved from viruses found in monkeys and chimpanzees—has been pandemic since at least the 1980s.<ref name="pmid29460740">{{cite journal |vauthors=Eisinger RW, Fauci AS |title=Ending the HIV/AIDS Pandemic1 |journal=Emerging Infectious Diseases |volume=24 |issue=3 |pages=413–416 |date=March 2018 |pmid=29460740 |pmc=5823353 |doi=10.3201/eid2403.171797 }}</ref> During the 20th century there were four pandemics caused by influenza virus and those that occurred in [[Spanish flu|1918]], [[1957–1958 influenza pandemic|1957]] and [[Hong Kong flu|1968]] were severe.<ref name="pmid30180422">{{cite journal |vauthors=Qin Y, Zhao MJ, Tan YY, Li XQ, Zheng JD, Peng ZB, Feng LZ |title=[History of influenza pandemics in China during the past century] |language=zh |journal=Zhonghua Liu Xing Bing Xue Za Zhi = Zhonghua Liuxingbingxue Zazhi |volume=39 |issue=8 |pages=1028–1031 |date=August 2018 |doi=10.3760/cma.j.issn.0254-6450.2018.08.003 |doi-broken-date=1 July 2025 |pmid=30180422 }}</ref> Before its eradication, smallpox was a cause of pandemics for more than 3,000 years.<ref name="pmid26060873">{{cite journal |vauthors=Nishiyama Y, Matsukuma S, Matsumura T, Kanatani Y, Saito T |title=Preparedness for a smallpox pandemic in Japan: public health perspectives |journal=Disaster Medicine and Public Health Preparedness |volume=9 |issue=2 |pages=220–223 |date=April 2015 |pmid=26060873 |doi=10.1017/dmp.2014.157 |s2cid=37149836 }}</ref> Throughout history, human migration has aided the spread of pandemic infections; first by sea and in modern times also by air.<ref name="pmid30878442">{{cite journal |vauthors=Houghton F |title=Geography, global pandemics & air travel: Faster, fuller, further & more frequent |journal=Journal of Infection and Public Health |volume=12 |issue=3 |pages=448–449 |date=2019 |pmid=30878442 |doi=10.1016/j.jiph.2019.02.020 |pmc=7129534 }}</ref>
With the exception of smallpox, most pandemics are caused by newly evolved viruses. These [[Emergent virus|"emergent"]] viruses are usually mutants of less harmful viruses that have circulated previously either in humans or in other animals.<ref>{{Cite web|url=https://virologyj.biomedcentral.com/articles/sections/emerging-viruses|title=Virology Journal|website=Virology Journal}}</ref>
[[Severe acute respiratory syndrome]] (SARS) and [[Middle East respiratory syndrome]] (MERS) are caused by new types of [[coronavirus]]es. Other coronaviruses are known to cause mild infections in humans,<ref name="pmid22094080">{{cite book |vauthors=Weiss SR, Leibowitz JL |title=Coronavirus pathogenesis |volume=81|pages=85–164 |year=2011 |pmid=22094080 |doi=10.1016/B978-0-12-385885-6.00009-2 |series=Advances in Virus Research |pmc=7149603 |isbn=978-0-12-385885-6}}</ref> so the virulence and rapid spread of SARS infections—that by July 2003 had caused around 8,000 cases and 800 deaths—was unexpected and most countries were not prepared.<ref name="pmid28475794">{{cite journal |vauthors=Wong AT, Chen H, Liu SH, Hsu EK, Luk KS, Lai CK, Chan RF, Tsang OT, Choi KW, Kwan YW, Tong AY, Cheng VC, Tsang DC |title=From SARS to Avian Influenza Preparedness in Hong Kong |journal=Clinical Infectious Diseases |volume=64 |issue=suppl_2 |pages=S98–S104 |date=May 2017 |pmid=28475794 |doi=10.1093/cid/cix123 |doi-access=free }}</ref>
A related coronavirus emerged in [[Wuhan]], China, in November 2019 and spread rapidly around the world. Thought to have originated in bats and subsequently named [[severe acute respiratory syndrome coronavirus 2]], infections with the virus cause a disease called [[COVID-19]], that varies in severity from mild to deadly,<ref name="WHOReport24Feb2020">{{cite report | title = Report of the WHO-China Joint Mission on Coronavirus Disease 2019 (COVID-19) | date = 16–24 February 2020 | url = https://www.who.int/docs/default-source/coronaviruse/who-china-joint-mission-on-covid-19-final-report.pdf | publisher = [[World Health Organization]] (WHO) | access-date = 21 March 2020}}</ref> and led to a [[COVID-19 pandemic|pandemic in 2020]].<ref name="pmid32143502"/><ref name="pmid32093211">{{cite journal |vauthors=Deng SQ, Peng HJ |title=Characteristics of and Public Health Responses to the Coronavirus Disease 2019 Outbreak in China |journal=Journal of Clinical Medicine |volume=9 |issue=2 |pages= 575|date=February 2020 |pmid=32093211 |doi=10.3390/jcm9020575 |pmc=7074453 |doi-access=free }}</ref><ref name="pmid32109444">{{cite journal |vauthors=Han Q, Lin Q, Jin S, You L |title=Coronavirus 2019-nCoV: A brief perspective from the front line |journal=The Journal of Infection |volume= 80|issue= 4|pages= 373–377|date=February 2020 |pmid=32109444 |doi=10.1016/j.jinf.2020.02.010 |pmc=7102581 }}</ref> Restrictions unprecedented in peacetime were placed on international travel,<ref>{{Cite news|url=https://www.nytimes.com/article/coronavirus-travel-restrictions.html|title=Coronavirus Travel Restrictions, Across the Globe| vauthors = Londoño E, Ortiz A |work=The New York Times |date=16 March 2020|via=NYTimes.com}}</ref> and [[curfews]] imposed in several major cities worldwide.<ref>{{Cite web|url=http://www.cidrap.umn.edu/news-perspective/2020/03/us-takes-more-big-pandemic-response-steps-europe-covid-19-cases-soar|title=US takes more big pandemic response steps; Europe COVID-19 cases soar|website=CIDRAP|date=15 March 2020 }}</ref>
=== In plants ===
{{Main|Plant pathology}}
[[File:Pepper mild mottle virus.png|thumb|125px|[[Capsicum|Peppers]] infected by [[pepper mild mottle virus]]]]
There are many types of [[plant virus]], but often they only cause a decrease in [[crop yield|yield]], and it is not economically viable to try to control them. Plant viruses are frequently spread from plant to plant by organisms called "[[Vector (epidemiology)|vectors]]". These are normally insects, but some [[fungi]], [[nematode]] worms and [[protozoa|single-celled organisms]] have also been shown to be vectors. When control of plant virus infections is considered economical (perennial fruits, for example) efforts are concentrated on killing the vectors and removing alternate hosts such as weeds.<ref>{{harvnb|Shors|2017|p=822}}</ref> Plant viruses are harmless to humans and other animals because they can only reproduce in living plant cells.<ref>{{harvnb|Shors|2017|pp=802–803}}</ref>
=== Bacteriophages ===
{{Main|Bacteriophage}}
[[File:PhageExterior.svg|thumb|upright=0.6|The structure of a typical bacteriophage]]
Bacteriophages are viruses that infect bacteria and [[archaea]].{{sfn | Oxford |Kellam|Collier| 2016 | p=19}} They are important in [[marine ecology]]: as the infected bacteria burst, carbon compounds are released back into the environment, which stimulates fresh organic growth. Bacteriophages are useful in scientific research because they are harmless to humans and can be studied easily. These viruses can be a problem in industries that produce food and drugs by [[Industrial fermentation|fermentation]] and depend on healthy bacteria. Some bacterial infections are becoming difficult to control with antibiotics, so there is a growing interest in [[phage therapy]], the use of bacteriophages to treat infections in humans.<ref>{{harvnb|Shors|2017|p=803}}</ref>
=== Host resistance ===
==== Innate immunity of animals ====
{{Main|Innate immunity}}
Animals, including humans, have many natural defences against viruses. Some are non-specific and protect against many viruses regardless of the type. This [[innate]] immunity is not improved by repeated exposure to viruses and does not retain a "memory" of the infection. The skin of animals, particularly its surface, which is made from dead cells, prevents many types of viruses from infecting the host. The acidity of the contents of the stomach
==== Adaptive immunity of animals ====
{{
[[
Specific immunity to viruses develops over time and white blood cells called [[lymphocyte]]s play a central role. Lymphocytes retain a "memory" of virus infections and produce many special molecules called [[antibody|antibodies]]. These antibodies attach to viruses and stop the virus from infecting cells. Antibodies are highly selective and attack only one type of virus. The body makes many different antibodies, especially during the initial infection. After the infection subsides, some antibodies remain and continue to be produced, usually giving the host lifelong immunity to the virus.<ref>{{harvnb|Shors|2017|pp=225–233}}</ref>
==== Plant resistance ====
Plants have elaborate and effective defence mechanisms against viruses. One of the most effective is the presence of so-called [[Gene-for-gene relationship#Classes of resistance gene|resistance (R) genes]]. Each R gene confers resistance to a particular virus by triggering localised areas of cell death around the infected cell, which can often be seen with the unaided eye as large spots. This stops the infection from spreading.<ref name="pmid30201857">{{cite journal |vauthors=Garcia-Ruiz H |title=Susceptibility Genes to Plant Viruses |journal=Viruses |volume=10 |issue=9 |year= 2018 |pages=484 |pmid=30201857 |pmc=6164914 |doi=10.3390/v10090484|doi-access=free }}</ref> RNA interference is also an effective defence in plants.<ref>{{harvnb|Shors|2017|p=812}}</ref> When they are infected, plants often produce natural disinfectants that destroy viruses, such as [[salicylic acid]], [[nitric oxide]] and [[Reactive oxygen species|reactive oxygen molecules]].<ref>{{cite journal | doi = 10.1038/nrmicro1239 | vauthors = Soosaar JL, Burch-Smith TM, Dinesh-Kumar SP | year = 2005 | title = Mechanisms of plant resistance to viruses | journal = Nature Reviews Microbiology | volume = 3 | issue = 10| pages = 789–798 | pmid = 16132037 | s2cid = 27311732 }}</ref>
==== Resistance to bacteriophages ====
The major way bacteria defend themselves from bacteriophages is by producing enzymes which destroy foreign DNA. These enzymes, called [[restriction endonucleases]], cut up the viral DNA that bacteriophages inject into bacterial cells.<ref name="pmid20056882">{{cite journal |vauthors=Horvath P, Barrangou R |s2cid=17960960 |title=CRISPR/Cas, the immune system of bacteria and archaea |journal=Science |volume=327 |issue=5962 |pages=167–170 |date=January 2010 |pmid=20056882 |doi=10.1126/science.1179555 |bibcode=2010Sci...327..167H |url=http://pdfs.semanticscholar.org/68e2/3e8e0dc19983b1f81b4be706587d0406ce36.pdf |archive-url=https://web.archive.org/web/20200327080505/http://pdfs.semanticscholar.org/68e2/3e8e0dc19983b1f81b4be706587d0406ce36.pdf |url-status=dead |archive-date=2020-03-27 }}</ref>
=== Prevention and treatment of viral disease ===
==== Vaccines ====
{{Details|Vaccination}}
[[File:DNA chemical structure.svg|thumb|The structure of DNA showing the position of the nucleosides and the phosphorus atoms that form the "backbone" of the molecule]]
Vaccines simulate a natural infection and its associated immune response, but do not cause the disease. Their use has resulted in the eradication of [[smallpox]] and a dramatic decline in illness and death caused by infections such as [[polio]], [[measles]], [[mumps]] and [[rubella]].<ref>{{harvnb|Shors|2017|pp=237–255}}</ref> Vaccines are available to prevent over fourteen viral infections of humans<ref name="pmid22003377">{{cite journal |vauthors=Small JC, Ertl HC |title=Viruses – from pathogens to vaccine carriers |journal=Current Opinion in Virology |volume=1 |issue=4 |year=2011 |pages=241–245 |pmid=22003377 |pmc=3190199 |doi=10.1016/j.coviro.2011.07.009 }}</ref> and more are used to prevent viral infections of animals.<ref name="pmid28618246">{{cite journal |vauthors=Burakova Y, Madera R, McVey S, Schlup JR, Shi J |title=Adjuvants for Animal Vaccines |journal=Viral Immunology |volume=31 |issue=1 |pages=11–22 |year=2018 |pmid=28618246 |doi=10.1089/vim.2017.0049 }}</ref> Vaccines may consist of either live or killed viruses.<ref name="auto">{{harvnb|Shors|2017|p=237}}</ref> Live vaccines contain weakened forms of the virus, but these vaccines can be dangerous when given to [[Immunodeficiency|people with weak immunity]]. In these people, the weakened virus can cause the original disease.<ref>{{cite journal |vauthors = Thomssen R | year = 1975 | title = Live attenuated versus killed virus vaccines | journal = Monographs in Allergy | volume = 9 | pages = 155–176 | pmid = 1090805 }}</ref> Biotechnology and genetic engineering techniques are used to produce "designer" vaccines that only have the capsid proteins of the virus. Hepatitis B vaccine is an example of this type of vaccine.<ref>{{harvnb|Shors|2017|p=238}}</ref> These vaccines are safer because they can never cause the disease.<ref name="auto" />
==== Antiviral drugs ====
{{Main|Antiviral drug}}
[[File:Guanosine aciclovir comparison.svg|thumb|upright|The structure of the DNA base [[guanosine]] and the antiviral drug [[aciclovir]] which functions by mimicking it]]
Since the mid-1980s, the development of [[antiviral drug]]s has increased rapidly, mainly driven by the AIDS pandemic. Antiviral drugs are often [[nucleoside analogue]]s, which masquerade as DNA building blocks ([[nucleoside]]s). When the replication of virus DNA begins, some of the fake building blocks are used. This prevents DNA replication because the drugs lack the essential features that allow the formation of a DNA chain. When DNA production stops the virus can no longer reproduce.<ref>{{harvnb|Shors|2017|pp=514–515}}</ref> Examples of nucleoside analogues are [[aciclovir]] for [[Herpesviridae|herpes virus]] infections and [[lamivudine]] for HIV and [[hepatitis B virus]] infections. Aciclovir is one of the oldest and most frequently prescribed antiviral drugs.<ref>{{harvnb|Shors|2017|p=514}}</ref>
Other antiviral drugs target different stages of the viral life cycle. HIV is dependent on an enzyme called the [[HIV-1 protease]] for the virus to become infectious. There is a class of drugs called [[protease inhibitors]], which bind to this enzyme and stop it from functioning.<ref name="{{harvnb|Shors|2017|p=463}}">{{harvnb|Shors|2017|p=568}}</ref>
Hepatitis C is caused by an RNA virus. In 80% of those infected, the disease becomes [[Chronic (medical)|chronic]], and they remain infectious for the rest of their lives unless they are treated. There are effective treatments that use [[direct-acting antivirals]].<ref name="pmid28319996">{{cite journal |vauthors=Falade-Nwulia O, Suarez-Cuervo C, Nelson DR, Fried MW, Segal JB, Sulkowski MS |title=Oral Direct-Acting Agent Therapy for Hepatitis C Virus Infection: A Systematic Review |journal=Annals of Internal Medicine |volume=166 |issue=9 |pages=637–648 |date=May 2017 |pmid=28319996 |pmc=5486987 |doi=10.7326/M16-2575}}</ref> Treatments for chronic carriers of the hepatitis B virus have been developed by a similar strategy, using lamivudine and other anti-viral drugs. In both diseases, the drugs stop the virus from reproducing and the interferon kills any remaining infected cells.<ref name="pmid21654909">{{cite journal | vauthors = Paul N, Han SH | title = Combination Therapy for Chronic Hepatitis B: Current Indications | journal = Curr Hepat Rep | volume = 10 | issue = 2 | pages = 98–105 | date = June 2011 | pmid = 21654909 | pmc = 3085106 | doi = 10.1007/s11901-011-0095-1 }}</ref>
HIV infections are usually treated with a combination of antiviral drugs, each targeting a different stage in the virus's life cycle. There are drugs that prevent the virus from attaching to cells, others that are nucleoside analogues and some poison the virus's enzymes that it needs to reproduce. The success of these drugs is proof of the importance of knowing how viruses reproduce.<ref name="{{harvnb|Shors|2017|p=463}}" />
== Role in ecology ==
Viruses are the most abundant biological entity in aquatic environments;<ref name="pmid16984643">{{cite journal | vauthors = Koonin EV, Senkevich TG, Dolja VV | title = The ancient Virus World and evolution of cells | journal = Biol. Direct | volume = 1 | page= 29 | date = September 2006 | pmid = 16984643 | pmc = 1594570 | doi = 10.1186/1745-6150-1-29 | doi-access = free }}</ref> one teaspoon of seawater contains about ten million viruses,<ref name="pmid31749771">{{cite journal |vauthors=Dávila-Ramos S, Castelán-Sánchez HG, Martínez-Ávila L, Sánchez-Carbente MD, Peralta R, Hernández-Mendoza A, Dobson AD, Gonzalez RA, Pastor N, Batista-García RA |title=A Review on Viral Metagenomics in Extreme Environments |journal=Frontiers in Microbiology |volume=10 |pages=2403 |date=2019 |pmid=31749771 |pmc=6842933 |doi=10.3389/fmicb.2019.02403 |doi-access=free }}</ref> and they are essential to the regulation of saltwater and freshwater ecosystems.<ref>{{harvnb|Shors|2017|p=5}}</ref> Most are bacteriophages,<ref name="pmid29867096">{{cite journal |vauthors=Breitbart M, Bonnain C, Malki K, Sawaya NA |s2cid=46927784 |title=Phage puppet masters of the marine microbial realm |journal=Nature Microbiology |volume=3 |issue=7 |pages=754–766 |date=July 2018 |pmid=29867096 |doi=10.1038/s41564-018-0166-y }}</ref> which are harmless to plants and animals. They infect and destroy the bacteria in aquatic microbial communities and this is the most important mechanism of recycling carbon in the marine environment. The organic molecules released from the bacterial cells by the viruses stimulate fresh bacterial and algal growth.<ref>{{harvnb|Shors|2017|pp=25–26}}</ref>
Microorganisms constitute more than 90% of the biomass in the sea. It is estimated that viruses kill approximately 20% of this biomass each day and that there are fifteen times as many viruses in the oceans as there are bacteria and archaea. They are mainly responsible for the rapid destruction of harmful [[algal bloom]]s,<ref name="pmid16163346">{{cite journal | vauthors = Suttle CA | title = Viruses in the sea | journal = Nature | volume = 437 | issue = 7057 | pages = 356–361 | date = September 2005 | pmid = 16163346 | doi = 10.1038/nature04160 |bibcode = 2005Natur.437..356S | s2cid = 4370363 }}</ref> which often kill other marine life.<ref>{{cite web|url=https://www.cdc.gov/hab/redtide/|title=Harmful Algal Blooms: Red Tide: Home | CDC HSB|publisher=www.cdc.gov|access-date=23 August 2009}}</ref>
The number of viruses in the oceans decreases further offshore and deeper into the water, where there are fewer host organisms.<ref name="pmid17853907">{{cite journal | vauthors = Suttle CA | title = Marine viruses – major players in the global ecosystem | journal = Nat. Rev. Microbiol. | volume = 5 | issue = 10 | pages = 801–812 | date = October 2007 | pmid = 17853907 | doi = 10.1038/nrmicro1750 | s2cid = 4658457 }}</ref>
Their effects are far-reaching; by increasing the amount of respiration in the oceans, viruses are indirectly responsible for reducing the amount of carbon dioxide in the atmosphere by approximately 3 [[gigatonne]]s of carbon per year.<ref name="pmid17853907" />
Marine mammals are also susceptible to viral infections. In 1988 and 2002, thousands of harbour seals were killed in Europe by [[phocine distemper virus]].<ref>{{cite journal | vauthors = Hall A, Jepson P, Goodman S, Harkonen T | title= Phocine distemper virus in the North and European Seas – Data and models, nature and nurture | journal= Biological Conservation | volume= 131 | issue= 2 | pages= 221–229 | year= 2006 |doi = 10.1016/j.biocon.2006.04.008 | bibcode= 2006BCons.131..221H }}</ref> Many other viruses, including [[caliciviruses]], [[Herpesviridae|herpesviruses]], [[adenovirus]]es and [[parvovirus]]es, circulate in marine mammal populations.<ref name="pmid17853907" />
Viruses can also serve as an alternative food source for microorganisms which engage in [[Virovore|virovory]], supplying nucleic acids, nitrogen, and phosphorus through their consumption.<ref name="New Virovore">{{Cite journal |last1=DeLong |first1=John P. |last2=Van Etten |first2=James L. |last3=Al-Ameeli |first3=Zeina |last4=Agarkova |first4=Irina V. |last5=Dunigan |first5=David D. |date=2023-01-03 |title=The consumption of viruses returns energy to food chains |journal=Proceedings of the National Academy of Sciences |language=en |volume=120 |issue=1 |pages=e2215000120 |doi=10.1073/pnas.2215000120 |doi-access=free |pmid=36574690 |pmc=9910503 |bibcode=2023PNAS..12015000D |issn=0027-8424}}</ref><ref name="First Virovore">{{cite news |last1=Irving |first1=Michael |title=First "virovore" discovered: An organism that eats viruses |url=https://newatlas.com/science/first-virovore-eats-viruses/ |access-date=29 December 2022 |publisher=New Atlas |date=28 December 2022 |archive-url=https://web.archive.org/web/20221229023549/https://newatlas.com/science/first-virovore-eats-viruses/ |archive-date=29 December 2022}}</ref>
== See also ==
* {{Portal inline|Medicine}}
* {{Portal inline|Viruses}}
== References ==
=== Notes ===
{{Reflist|30em}}
=== Bibliography ===
{{Refbegin}}
*{{cite book | editor-last = Collier | editor-first =Leslie |editor-last2=Balows| editor-first2 =Albert | editor-last3 =Sussman | editor-first3 =Max | title = Topley & Wilson's Microbiology and Microbial Infections | publisher = Arnold | year = 1998 | isbn = 0-340-66316-2 |edition=9th|volume=1, ''Virology''}}
* {{cite book | last1=Howley | first1=Peter M. | last2=Knipe | first2=David M. | last3=Enquist | first3=Lynn W. | title=Fields Virology: Fundamentals | publisher=LWW | publication-place=Philadelphia | date=2023-09-25 | isbn=978-1-9751-1251-6}}
*{{cite book | last1=Oxford
| first1=John |last2=Kellam|first2=Paul|last3=Collier|first3=Leslie|
title=Human Virology | publisher=Oxford University Press | publication-place=Oxford | year=2016 | isbn=978-0-19-871468-2 | oclc=968152575}}
*{{cite book | last = Shors | first = Teri | title = Understanding Viruses | publisher = Jones and Bartlett Publishers | year = 2017 | isbn = 978-1284025927 }}
{{Refend}}
== External links ==
{{Library resources box
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|lcheading= Viruses
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}}
* [https://www.viprbrc.org/brc/home.spg?decorator=vipr Virus Pathogen Resource] – Genomic and other research data about human pathogenic viruses
* [https://www.fludb.org/brc/home.spg?decorator=influenza Influenza Research Database]{{snd}}Genomic and other research data about influenza viruses
{{Virus topics}}
{{Introductory science articles}}
{{DEFAULTSORT:Viruses, Introduction to}}
[[Category:Virology|*]]
[[Category:Viruses|
[[simple:Virus]]
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