Introduction to viruses: Difference between revisions

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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" ({{lang-la|text= [[contagium vivum fluidum]]}})—or a "soluble living germ" because he could not find any germ-like particles.<ref>{{harvnb|Collier|Balows|Sussman|1998|p=3}}</ref> 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>

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== Origins ==

{{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>

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==== 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 }}</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 the 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 |pages=103 |date=February 2020 |pmid=32103776 |pmc=7045448 |doi=10.1186/s13071-020-3985-7 |doi-access=free }}</ref>

==== Pandemic and emergent ====