Editing Polio (section)

== Cause ==
{{main|Poliovirus}}
[[File:Polio EM PHIL 1875 lores.PNG|thumb|right|A [[Transmission electron microscopy|transmission electron microscope]] [[micrograph]] of poliovirus]]
Poliomyelitis does not affect any species other than humans.<ref name="Sherris">{{cite book |url=https://archive.org/details/sherrismedicalmi00ryan |title=Sherris Medical Microbiology |publisher=McGraw Hill |year=2004 |isbn=978-0-8385-8529-0 |veditors=Ryan KJ, Ray CG |edition=4th |pages=[https://archive.org/details/sherrismedicalmi00ryan/page/n552 535]–37 |chapter=Enteroviruses |url-access=limited}}</ref> The disease is caused by infection with a member of the [[genus]] ''[[Enterovirus]]'' known as [[poliovirus]] (PV). This group of [[RNA virus]]es colonize the [[Human gastrointestinal tract|gastrointestinal tract]]<ref name=Harrison>{{cite book| author = Cohen JI| chapter = Chapter 175: Enteroviruses and Reoviruses| title = Harrison's Principles of Internal Medicine| veditors = [[Dennis Kasper|Kasper DL]], [[Eugene Braunwald|Braunwald E]], [[Anthony Fauci|Fauci AS]], etal | edition = 16th | publisher = McGraw-Hill Professional| year = 2004| page = 1144| isbn = 978-0-07-140235-4 | title-link = Harrison's Principles of Internal Medicine}}</ref>&nbsp;– specifically the [[oropharynx]] and the [[intestine]]. Its [[Virus structure|structure]] is quite simple, composed of a single [[sense (molecular biology)|(+) sense]] [[RNA]] [[genome]] enclosed in a protein shell called a [[capsid]].<ref name=Sherris /> In addition to protecting the virus' genetic material, the capsid proteins enable poliovirus to infect certain types of cells. Three [[serovar|serotypes]] of poliovirus have been identified&nbsp;– wild poliovirus type 1 (WPV1), type 2 (WPV2), and type 3 (WPV3)&nbsp;– each with a slightly different capsid protein.<ref>{{cite book | vauthors = Katz SL, Gershon AA, [[Saul Krugman|Krugman S]], Hotez PJ |title=Krugman's infectious diseases of children |url=https://archive.org/details/krugmansinfectio00gers_995 |url-access=limited |publisher=Mosby |location=St. Louis |year=2004 |pages=[https://archive.org/details/krugmansinfectio00gers_995/page/n220 81]–97 |isbn=978-0-323-01756-5 }}</ref> All three are extremely [[virulence|virulent]] and produce the same disease symptoms.<ref name=Sherris /> WPV1 is the most commonly encountered form, and the one most closely associated with paralysis.<ref name="Ohri">{{cite journal |vauthors=Ohri LK, Marquess JG |year=1999 |title=Polio: Will We Soon Vanquish an Old Enemy? |url=http://www.medscape.com/viewarticle/416890 |journal=Drug Benefit Trends |volume=11 |issue=6 |pages=41–54 |archive-url=https://web.archive.org/web/20040204232019/http://www.medscape.com/viewarticle/416890 |archive-date=4 February 2004 |access-date=23 August 2008}} (Available free on [[Medscape]]; registration required.)</ref> WPV2 was certified as eradicated in 2015 and WPV3 certified as eradicated in 2019.<ref>{{Cite web |title=Two out of three wild poliovirus strains eradicated |url=https://www.who.int/news-room/feature-stories/detail/two-out-of-three-wild-poliovirus-strains-eradicated |access-date=2022-08-28 |website=www.who.int |archive-date=30 October 2019 |archive-url=https://web.archive.org/web/20191030210736/https://www.who.int/news-room/feature-stories/detail/two-out-of-three-wild-poliovirus-strains-eradicated |url-status=live }}</ref>

The [[incubation period]] (from exposure to the first signs and symptoms) ranges from three to six days for nonparalytic polio. If the disease progresses to cause paralysis, this occurs within 7 to 21 days.<ref name="PinkBook2021" />

Individuals who are exposed to the virus, either through infection or by [[immunization]] via [[polio vaccine]], develop [[immunity (medical)|immunity]]. In immune individuals, [[IgA]] [[antibodies]] against poliovirus are present in the [[tonsil]]s and [[gastrointestinal tract]] and able to block [[Viral replication|virus replication]]; [[Immunoglobulin G|IgG]] and [[Immunoglobulin M|IgM]] antibodies against PV can prevent the spread of the virus to motor neurons of the [[central nervous system]].<ref name="Kew_2005">{{cite journal |vauthors=Kew OM, Sutter RW, de Gourville EM, Dowdle WR, Pallansch MA |year=2005 |title=Vaccine-derived polioviruses and the endgame strategy for global polio eradication |url=https://zenodo.org/record/1235033 |journal=Annual Review of Microbiology |volume=59 |pages=587–635 |doi=10.1146/annurev.micro.58.030603.123625 |pmid=16153180 |access-date=30 June 2019 |archive-date=9 July 2020 |archive-url=https://web.archive.org/web/20200709053801/https://zenodo.org/record/1235033 |url-status=live }}</ref> Infection or vaccination with one serotype of poliovirus does not provide immunity against the other serotypes, and full immunity requires exposure to each serotype.<ref name=Kew_2005/>

A rare condition with a similar presentation, nonpoliovirus poliomyelitis, may result from infections with [[enterovirus]]es other than poliovirus.<ref>{{cite journal | vauthors = Gorson KC, Ropper AH | title = Nonpoliovirus poliomyelitis simulating Guillain-Barré syndrome | journal = Archives of Neurology | volume = 58 | issue = 9 | pages = 1460–64 | date = September 2001 | pmid = 11559319 | doi = 10.1001/archneur.58.9.1460 | doi-access =  }}</ref>

The oral polio vaccine, which has been in use since 1961,<ref name="WHO2016">{{cite journal |vauthors=((World Health Organization)) |date=2016 |title=Polio vaccines: WHO position paper – March, 2016 |journal=Weekly Epidemiological Record |volume=91 |issue=12 |pages=145–68 |pmid=27039410 |hdl-access=free |hdl=10665/254399}}</ref> contains [[Attenuated vaccine|weakened]] viruses that can replicate. On rare occasions, these may be transmitted from the vaccinated person to other people; in communities with [[Herd immunity|good vaccine coverage]], transmission is limited, and the virus dies out. In communities with low vaccine coverage, this weakened virus may continue to circulate and, over time may mutate and revert to a virulent form.<ref>{{Cite web |title=What is vaccine-derived polio? |url=https://www.gavi.org/vaccineswork/what-vaccine-derived-polio |access-date=2022-08-28 |website=www.gavi.org |archive-date=28 August 2022 |archive-url=https://web.archive.org/web/20220828160534/https://www.gavi.org/vaccineswork/what-vaccine-derived-polio |url-status=live }}</ref> Polio arising from this cause is referred to as ''circulating vaccine-derived poliovirus'' (cVDPV) or ''variant poliovirus'' in order to distinguish it from the natural or "wild" poliovirus (WPV).<ref>{{Cite web |title=GPEI-Vaccine-Derived Polioviruses |url=https://polioeradication.org/polio-today/polio-prevention/the-virus/vaccine-derived-polio-viruses/ |access-date=2022-08-28 |archive-date=28 August 2022 |archive-url=https://web.archive.org/web/20220828160534/https://polioeradication.org/polio-today/polio-prevention/the-virus/vaccine-derived-polio-viruses/ |url-status=live }}</ref>

=== Transmission ===
Poliomyelitis is highly contagious. The disease is [[Transmission (medicine)|transmitted]] primarily via the [[fecal–oral route]], by ingesting contaminated food or water. It is occasionally transmitted via the oral–oral route.<ref name="PinkBook2021" /> It is seasonal in [[temperate climate]]s, with peak transmission occurring in summer and autumn. These seasonal differences are far less pronounced in [[tropical climate|tropical]] areas.<ref name="Kew_2005" /> Polio is most infectious between 7 and 10 days before and after the appearance of symptoms, but transmission is possible as long as the virus remains in the saliva or feces. Virus particles can be  excreted in the [[feces]] for up to six weeks.<ref name="ECDC" />

Factors that increase the risk of polio infection include [[pregnancy]],<ref name="Evans_1960">{{cite journal |vauthors=Evans CA |date=December 1960 |title=Factors influencing the occurrence of illness during naturally acquired poliomyelitis virus infections |journal=Bacteriological Reviews |volume=24 |issue=4 |pages=341–52 |doi=10.1128/MMBR.24.4.341-352.1960 |pmc=441061 |pmid=13697553}}</ref> being very old or very young, [[immune deficiency]],<ref>{{Citation |title=Polio and the late effects of polio |url=https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/polio-and-post-polio-syndrome |access-date=2023-04-12 |publisher=Department of Health, State Government of Victoria, Australia |archive-date=12 April 2023 |archive-url=https://web.archive.org/web/20230412211550/https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/polio-and-post-polio-syndrome |url-status=live }}</ref> and [[malnutrition]].<ref>{{cite journal | vauthors = Chandra RK | title = Reduced secretory antibody response to live attenuated measles and poliovirus vaccines in malnourished children | journal = British Medical Journal | volume = 2 | issue = 5971 | pages = 583–5 | date = June 1975 | pmid = 1131622 | pmc = 1673535 | doi = 10.1136/bmj.2.5971.583 }}</ref> Although the virus can cross the [[placenta|maternal-fetal barrier]] during pregnancy, the fetus does not appear to be affected by either maternal infection or polio vaccination.<ref name=UK>{{cite book |veditors=((Joint Committee on Vaccination and Immunisation)), Salisbury A, Ramsay M, Noakes K |title=Chapter 26:Poliomyelitis. ''in:'' Immunisation Against Infectious Disease, 2006 |url=http://www.immunisation.nhs.uk/files/GB_26_polio.pdf |publisher=[[Office of Public Sector Information|Stationery Office]] |location=Edinburgh |year=2006 |pages=313–29 |isbn=978-0-11-322528-6 |archive-url=https://web.archive.org/web/20090326085852/http://www.immunisation.nhs.uk/files/GB_26_polio.pdf |archive-date=26 March 2009 |access-date=9 March 2007 }}</ref> Maternal antibodies also cross the [[placenta]], providing [[passive immunity]] that protects the infant from polio infection during the first few months of life.<ref>{{cite journal | vauthors = Sauerbrei A, Groh A, Bischoff A, Prager J, Wutzler P | title = Antibodies against vaccine-preventable diseases in pregnant women and their offspring in the eastern part of Germany | journal = Medical Microbiology and Immunology | volume = 190 | issue = 4 | pages = 167–72 | date = March 2002 | pmid = 12005329 | doi = 10.1007/s00430-001-0100-3 | s2cid = 12369344 | doi-access = free }}</ref>