Editing Reassortment (section)

== Flu virus ==
The classical example of reassortment is seen in the [[Orthomyxoviridae|influenza viruses]], whose genomes consist of eight distinct segments of RNA.  These segments act like mini-chromosomes, and each time a flu virus is assembled, it requires one copy of each segment.

If a single host (a human, a chicken, or other animal) is infected by two different strains of the influenza virus, then it is possible that new assembled viral particles will be created from segments whose origin is mixed, some coming from one strain and some coming from another. The new reassortant strain will share properties of both of its parental lineages.

Reassortment is responsible for some of the major [[antigenic shift]]s in the history of the influenza virus. In the 1957 "[[1957–1958 influenza pandemic|Asian flu]]" and 1968 "[[Hong Kong flu]]" [[pandemic]]s, flu strains were caused by reassortment between an avian virus and a human virus.<ref>{{Cite web|last=|first=|date=2019-01-22|title=1968 Pandemic (H3N2 virus)|url=https://www.cdc.gov/flu/pandemic-resources/1968-pandemic.html|archive-url=|archive-date=|access-date=2021-01-18|website=US Centers for Disease Control and Prevention (CDC)|language=en-us}}</ref><ref>{{Cite journal|last1=Saunders-Hastings|first1=Patrick R.|last2=Krewski|first2=Daniel|date=2016-12-06|title=Reviewing the History of Pandemic Influenza: Understanding Patterns of Emergence and Transmission|journal=Pathogens|volume=5|issue=4|page=66|doi=10.3390/pathogens5040066|issn=2076-0817|pmc=5198166|pmid=27929449|doi-access=free}}</ref> In addition, the [[H1N1]] virus responsible for the [[2009 swine flu pandemic]] has an unusual mix of swine, avian and human influenza genetic sequences.<ref name="NewSci-20090424-pandemic">{{cite news|url=https://www.newscientist.com/article/dn17025-deadly-new-flu-virus-in-us-and-mexico-may-go-pandemic.html|title=Deadly new flu virus in US and Mexico may go pandemic|work=[[New Scientist]]|date=2009-04-24|access-date=2009-04-26}}</ref>

===Multiplicity reactivation===

When [[influenza A virus|influenza viruses]] are inactivated by [[ultraviolet|UV]] irradiation or [[ionizing radiation]], they remain capable of multiplicity reactivation in infected host cells.<ref>Barry RD. The multiplication of influenza virus. II. Multiplicity reactivation of ultraviolet irradiated virus.  Virology. 1961 Aug;14:398-405.  {{PMID|13687359}} DOI: 10.1016/0042-6822(61)90330-0</ref><ref>Henle W, Liu OC. Studies on host-virus interactions in the chick embryo-influenza virus system. VI. Evidence for multiplicity reactivation of inactivated virus.  J Exp Med. 1951 Oct;94(4):305-22.  {{PMID|14888814}}</ref><ref>Gilker JC, Pavilanis V, Ghys R. Multiplicity reactivation in gamma irradiated influenza viruses. Nature. 1967 Jun 17;214(5094):1235-7. {{PMID|6066111}} DOI: 10.1038/2141235a0</ref>  If any of a virus's [[genome]] segments is damaged in such a way as to prevent replication or expression of an essential [[gene]], the virus is inviable when it, alone, infects a host cell (single infection).  However, when two or more damaged viruses infect the same cell (multiple infection), the infection can often succeed (multiplicity reactivation) due to reassortment of segments, provided that each of the eight genome segments is present in at least one undamaged copy.<ref>Michod RE, Bernstein H, Nedelcu AM.  Adaptive value of sex in microbial pathogens.  Infect Genet Evol. 2008 May;8(3):267-85. doi: 10.1016/j.meegid.2008.01.002. Epub 2008 Jan 16. Review.  {{PMID|18295550}}</ref>