Editing Epidemic (section)

=== Antigenic Change ===
[[File:Differences-Between-Antigenic-Shift-and-Antigenic-Drift.webp|thumb|Illustration depicting the different mechanisms of antigenic shift and antigenic drift]]
An [[antigen]] is a [[protein]] on the [[virus]]' surface that host [[Antibody|antibodies]] can recognize and attack. Changes in the [[Antigenic variation|antigenic characteristics]] of the agent make it easier for the changed virus to spread throughout a previously immune population. There are two natural mechanisms for change - [[antigenic drift]] and [[antigenic shift]]. '''Antigenic drift''' arises over a period of time as an accumulation of [[mutation]]s in the [[Virus#Genome|virus genes]], possibly through a series of hosts, and eventually gives rise to a new strain of virus which can evade existing immunity. '''Antigenic shift''' is abrupt - in this, two or more different strains of a [[virus]], [[Coinfection|coinfecting]] a single host, combine to form a new subtype having a mixture of characteristics of the original strains. The best known and best documented example of both processes is [[influenza]].<ref>{{Cite web |last=CDC |date=12 December 2022 |title=How Flu Viruses Can Change |url=https://www.cdc.gov/flu/about/viruses/change.htm |access-date=9 September 2023 |website=Centers for Disease Control and Prevention |language=en-us}}</ref> [[SARS-CoV-2|SARS-CoV2]] has demonstrated antigenic drift and possibly shift as well.<ref>{{Cite journal |last1=Carabelli |first1=Alessandro M. |last2=Peacock |first2=Thomas P. |last3=Thorne |first3=Lucy G. |last4=Harvey |first4=William T. |last5=Hughes |first5=Joseph |last6=de Silva |first6=Thushan I. |last7=Peacock |first7=Sharon J. |last8=Barclay |first8=Wendy S. |last9=de Silva |first9=Thushan I. |last10=Towers |first10=Greg J. |last11=Robertson |first11=David L. |date=March 2023 |title=SARS-CoV-2 variant biology: immune escape, transmission and fitness |journal=Nature Reviews Microbiology |language=en |volume=21 |issue=3 |pages=162–177 |doi=10.1038/s41579-022-00841-7 |pmid=36653446 |pmc=9847462 |issn=1740-1534}}</ref>