Introduction to viruses: Difference between revisions

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{{short description|Non-technical introduction to viruses}}

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== 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 journalbook | vauthors = Alwine JC | chapter = Modulation of Host Cell Stress Responses by Human Cytomegalovirus | title = Human Cytomegalovirus: Modulation of host cell stress responses by human cytomegalovirus | journal = Curr. Top. Microbiol. Immunol. | volume = 325 | pages = 263–279 | date = 2008 | pmid = 18637511 | doi = 10.1007/978-3-540-77349-8_15 | series = Current Topics in Microbiology and Immunology | 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>

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[[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">{{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><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 ===

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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&nbsp;– 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, herpesviruses, adenoviruses and parvoviruses, 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 |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>