View source for Epidemic

{{Short description|Rapid spread of disease affecting a large number of people in a short time}}
{{Other uses}}
{{cs1 config|name-list-style=vanc}}
[[File:2014 West Africa Ebola Epidemic - New Cases per Week.svg|thumb|upright=1.5|Example of an epidemic showing the number of new infections over time.]]
An '''epidemic''' (from [[Ancient Greek|Greek]] ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of [[disease]] to a large number of [[Host (biology)|hosts]] in a given population within a short period of time. For example, in [[meningococcal infection]]s, an [[attack rate]] in excess of 15 cases per 100,000 people for two consecutive weeks is considered an epidemic.<ref name="Principles of Epidemiology"/><ref name=Green>{{cite journal | vauthors = Green MS, Swartz T, Mayshar E, Lev B, Leventhal A, Slater PE, Shemer J | title = When is an epidemic an epidemic? | journal = The Israel Medical Association Journal | volume = 4 | issue = 1 | pages = 3–6 | date = January 2002 | pmid = 11802306 | url = https://www.ima.org.il/FilesUploadPublic/IMAJ/0/55/27606.pdf }}</ref>

Epidemics of infectious disease are generally caused by several factors including a change in the ecology of the host population (e.g., increased stress or increase in the density of a vector species), a genetic change in the pathogen reservoir or the introduction of an emerging pathogen to a host population (by movement of pathogen or host). Generally, an epidemic occurs when host [[Immunity (medicine)|immunity]] to either an established pathogen or newly emerging [[novel pathogen]] is suddenly reduced below that found in the [[Endemic (epidemiology)|endemic]] equilibrium and the transmission threshold is exceeded.<ref>{{cite book | veditors = Callow PP | chapter = Epidemic | chapter-url = https://books.google.com/books?id=JKTeiqPBpM4C&pg=PA246 | title = The Encyclopedia of Ecology and Environmental Management | publisher = Blackwell Science Ltd. | location = Oxford | date = 1998 | pages = 246 | isbn = 0-86542-838-7 }}</ref>

An epidemic may be restricted to one location; however, if it spreads to other countries or continents and affects a substantial number of people, it may be termed as a [[pandemic]].<ref name="Principles of Epidemiology">{{cite book |last1=Dicker |first1=Richard C. |url=https://stacks.cdc.gov/view/cdc/6914 |title=Principles of epidemiology in public health practice; an introduction to applied epidemiology and biostatistics. 3rd ed |last2=Coronado |first2=Fátima |last3=Koo |first3=Denise |last4=Parrish |first4=R. Gibson |publisher=Centers for Disease Control and Prevention |year=2012 |edition=Third |location=Atlanta, Georgia}}</ref>{{rp|§1:72|location=}} The declaration of an epidemic usually requires a good understanding of a [[baseline (medicine)|baseline]] rate of [[incidence (epidemiology)|incidence]]; epidemics for certain diseases, such as [[influenza]], are defined as reaching some defined increase in incidence above this baseline.<ref name=Green/> A few cases of a very rare [[disease]] may be classified as an epidemic, while many cases of a common disease (such as the [[common cold]]) would not. An epidemic can cause enormous damage through financial and economic losses in addition to impaired health and loss of life.{{citation needed|date=July 2023}}

==Definition==

[[File:Plague in an Ancient City LACMA AC1997.10.1 (1 of 2).jpg|thumb|upright=1.25|''The Plague of Athens'' ({{Circa|1652}}–1654) by [[Michiel Sweerts]], illustrating the [[Plague of Athens|devastating epidemic that struck Athens in 430 BC]], as described by the historian [[Thucydides]]]]

The United States [[Centers for Disease Control and Prevention]] defines epidemic broadly: "Epidemic refers to an increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area." The term "outbreak" can also apply, but is usually restricted to smaller events.<ref name="Principles of Epidemiology" />{{rp|§1:72|location=}}<ref name="Green" />

Any sudden increase in disease prevalence may generally be termed an epidemic. This may include [[contagious disease]] (i.e. easily spread between persons) such as [[influenza]]; [[Disease vector|vector-borne]] diseases such as [[malaria]]; [[Waterborne diseases|water-borne diseases]] such as [[cholera]]; and [[Sexually transmitted infection|sexually transmitted]] diseases such as [[HIV/AIDS]]. The term can also be used for non-communicable health issues such as [[obesity epidemic|obesity]].<ref name="Green" /><ref>[https://www.who.int/nutrition/topics/obesity/en/ Controlling the global obesity epidemic], the World Health Organization</ref><ref name="Martin" />

The term ''epidemic'' derives from a word form attributed to [[Homer]]'s ''[[Odyssey]]'', which later took its medical meaning from the ''[[:s:Of the Epidemics|Epidemics]],'' a treatise by [[Hippocrates]].<ref name="Martin">{{cite journal | vauthors = Martin PM, Martin-Granel E | title = 2,500-year evolution of the term epidemic | journal = Emerging Infectious Diseases | volume = 12 | issue = 6 | pages = 976–80 | date = June 2006 | pmid = 16707055 | pmc = 3373038 | doi = 10.3201/eid1206.051263 }}</ref> Before Hippocrates, {{Transliteration|grc|epidemios}}, {{Transliteration|grc|epidemeo}}, {{Transliteration|grc|epidamos}}, and other variants had meanings similar to the current definitions of "[[Wiktionary:indigenous|indigenous]]" or "[[Endemic (epidemiology)|endemic]]".<ref name="Martin" /> [[Thucydides]]' description of the [[Plague of Athens]] is considered one of the earliest accounts of a disease epidemic.<ref name="Martin" /> By the early 17th century, the terms ''endemic'' and ''epidemic'' referred to contrasting conditions of population-level disease, with the endemic condition a "''common sicknesse''" and the epidemic "''hapning in some region, or countrey, at a certaine time, ....... producing in all sorts of people, one and the same kind of sicknesse''".<ref name="Lodge1603">{{cite book |last=Lodge |first=Thomas |url=https://archive.org/details/b3032810x |title=A treatise of the plague: containing the nature, signes, and accidents of the same, with the certaine and absolute cure of the fevers, botches and carbuncles that raigne in these times |publisher=Edward White |year=1603 |location=London |chapter= |quote=CHAP. 1. Of the nature and essence of the Plague}}</ref>

The term "epidemic" is often applied to diseases in non-human animals, although "[[epizootic]]" is technically preferable.<ref>{{Cite news |last=McKie |first=Robin |date=2021-02-21 |title=Foot and mouth 20 years on: what an animal virus epidemic taught UK science |language=en-GB |work=The Observer |url=https://www.theguardian.com/science/2021/feb/21/foot-and-mouth-20-years-on-what-an-animal-virus-epidemic-taught-uk-science |access-date=2023-09-11 |issn=0029-7712}}</ref><ref>{{Cite web |date=11 December 2005 |title=Emergency response for epizootic diseases |url=https://www.afbini.gov.uk/articles/emergency-response-epizootic-diseases |access-date=11 September 2023 |website=Agri-Food and Biosciences Institute |language=en}}</ref>

==Causes==
There are several factors that may contribute (individually or in combination) to causing an epidemic. There may be changes in a [[pathogen]], in the population that it can infect, in the environment, or in the interaction between all three. Factors include the following:<ref name="Principles of Epidemiology"/>{{rp|§1:72|location=}}

=== 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>

=== Drug resistance ===
[[Antimicrobial resistance|Antibiotic resistance]] applies specifically to [[bacteria]] that become resistant to [[antibiotic]]s.<ref name="WHO2014">{{cite web |date=April 2014 |title=Antimicrobial resistance Fact sheet N°194 |url=https://www.who.int/mediacentre/factsheets/fs194/en/ |url-status=live |archive-url=https://web.archive.org/web/20150310081111/http://www.who.int/mediacentre/factsheets/fs194/en/ |archive-date=10 March 2015 |access-date=7 March 2015 |website=who.int}}</ref> Resistance in bacteria can arise naturally by [[genetic mutation]], or by one species acquiring resistance from another through [[horizontal gene transfer]].<ref>{{Cite web |title=General Background: About Antibiotic Resistance |url=http://www.tufts.edu/med/apua/about_issue/about_antibioticres.shtml |url-status=dead |archive-url=https://web.archive.org/web/20151023035356/http://www.tufts.edu/med/apua/about_issue/about_antibioticres.shtml |archive-date=23 October 2015 |access-date=30 October 2015 |website=www.tufts.edu}}</ref> Extended use of antibiotics appears to encourage selection for mutations which can render antibiotics ineffective. This is especially true of [[tuberculosis]], with increasing occurrence of [[Multi-drug-resistant tuberculosis|multiple drug-resistant tuberculosis]] (MDR-TB) worldwide.<ref name="WHO2015Fact">{{Cite web |title=Tuberculosis (TB) |url=https://www.who.int/news-room/fact-sheets/detail/tuberculosis |url-status=live |archive-url=https://web.archive.org/web/20200730165218/https://www.who.int/news-room/fact-sheets/detail/tuberculosis |archive-date=30 July 2020 |access-date=8 May 2020 |website=who.int |language=en}}</ref><ref>{{cite journal |vauthors=Dabour R, Meirson T, Samson AO |date=December 2016 |title=Global antibiotic resistance is mostly periodic |journal=Journal of Global Antimicrobial Resistance |volume=7 |pages=132–134 |doi=10.1016/j.jgar.2016.09.003 |pmid=27788414}}</ref>

=== Changes in transmission ===
[[File:Anopheles gambiae mosquito feeding 1354.p lores.jpg|thumb|Anopheles mosquito, the vector of malaria]]
[[Pathogen transmission]] is a term used to describe the mechanisms by which a disease-causing agent (virus, bacterium, or parasite) spreads from one host to another. Common modes of transmission include:<ref>{{Cite web |title=FAQ: Methods of Disease Transmission |url=https://eportal.mountsinai.ca/Microbiology/faq/transmission.shtml |access-date=10 January 2024 |website=Department of Microbiology, Mount Sinai Hospital}}</ref> -
* [[Airborne transmission|airborne]] (as with influenza and COVID-19),
* [[Fecal–oral route|fecal-oral]] (as with cholera and typhoid),
* [[vector-borne]] (malaria, Zika) and
* [[Sexually transmitted infection|sexual]] (syphilis, HIV)

The first three of these require that pathogen must survive away from its host for a period of time; an evolutionary change which increases survival time will result in increased virulence.<ref>{{Cite news |last=Mandavilli |first=Apoorva |date=1 October 2021 |title=Is the Coronavirus Getting Better at Airborne Transmission? |language=en-US |work=The New York Times |url=https://www.nytimes.com/2021/10/01/health/coronavirus-aerosols-airborne.html |access-date=12 September 2023 |issn=0362-4331}}</ref>

Another possibility, although rare, is that a pathogen may adapt to take advantage of a new mode of transmission<ref>{{Cite journal |last=Alcamí |first=Antonio |date=2023-03-28 |title=Pathogenesis of the circulating mpox virus and its adaptation to humans |journal=Proceedings of the National Academy of Sciences |language=en |volume=120 |issue=13 |pages=e2301662120 |doi=10.1073/pnas.2301662120 |issn=0027-8424 |pmc=10068839 |pmid=36940331|bibcode=2023PNAS..12001662A }}</ref><ref>{{Cite journal |last1=Antonovics |first1=Janis |last2=Wilson |first2=Anthony J. |last3=Forbes |first3=Mark R. |last4=Hauffe |first4=Heidi C. |last5=Kallio |first5=Eva R. |last6=Leggett |first6=Helen C. |last7=Longdon |first7=Ben |last8=Okamura |first8=Beth |last9=Sait |first9=Steven M. |last10=Webster |first10=Joanne P. |date=2017-05-05 |title=The evolution of transmission mode |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |language=en |volume=372 |issue=1719 |pages=20160083 |doi=10.1098/rstb.2016.0083 |issn=0962-8436 |pmc=5352810 |pmid=28289251}}</ref>

=== Seasonality ===
Seasonal diseases arise due to the change in the environmental conditions, especially such as humidity and temperature, during different seasons. Many diseases display [[seasonality]],<ref>{{Cite journal |last=Martinez |first=Micaela Elvira |date=8 November 2018 |title=The calendar of epidemics: Seasonal cycles of infectious diseases |journal=PLOS Pathogens |language=en |volume=14 |issue=11 |pages=e1007327 |doi=10.1371/journal.ppat.1007327 |issn=1553-7374 |pmc=6224126 |pmid=30408114 |doi-access=free }}</ref><ref>{{Cite web |date=8 November 2018 |title=Mark Your Calendar: All Infectious Diseases Are Seasonal |url=https://www.publichealth.columbia.edu/news/mark-your-calendar-all-infectious-diseases-are-seasonal |access-date=13 September 2023 |website=Columbia University Mailman School of Public Health |language=en}}</ref> This may be due to one or more of the following underlying factors: -<ref>{{Cite journal |last1=Grassly |first1=Nicholas C |last2=Fraser |first2=Christophe |date=7 October 2006 |title=Seasonal infectious disease epidemiology |journal=Proceedings of the Royal Society B: Biological Sciences |volume=273 |issue=1600 |pages=2541–2550 |doi=10.1098/rspb.2006.3604 |issn=0962-8452 |pmc=1634916 |pmid=16959647}}</ref>
* The ability of the pathogen to survive outside the host - e.g. [[Waterborne diseases|water-borne]] cholera<ref>{{Cite journal |last=Leitzell |first=Katherine |date=20 November 2011 |title=The Time of Cholera |url=https://www.earthdata.nasa.gov/learn/sensing-our-planet/the-time-of-cholera |journal=NASA Earthdata |language=en}}</ref> which becomes prevalent in [[Monsoon|tropical wet]] seasons, or influenza which peaks in temperate regions during winter.<ref>{{Cite web |last=CDC |date=2022-09-20 |title=Learn more about the flu season |url=https://t.cdc.gov/C03 |access-date=2023-09-13 |website=Centers for Disease Control and Prevention |language=en-us}}</ref><ref>{{Cite journal |last1=Marr |first1=Linsey C. |last2=Tang |first2=Julian W. |last3=Van Mullekom |first3=Jennifer|author3-link=Jennifer Van Mullekom |last4=Lakdawala |first4=Seema S. |date=January 2019 |title=Mechanistic insights into the effect of humidity on airborne influenza virus survival, transmission and incidence |journal=Journal of the Royal Society Interface |language=en |volume=16 |issue=150 |pages=20180298 |doi=10.1098/rsif.2018.0298 |issn=1742-5689 |pmc=6364647 |pmid=30958176}}</ref>
* The behaviour of people susceptible to the disease - such as spending more time in close contact indoors.<ref>{{Cite web |last=Robson |first=David |date=19 October 2015 |title=The real reason germs spread in the winter |url=https://www.bbc.com/future/article/20151016-the-real-reason-germs-spread-in-the-winter |access-date=14 September 2023 |website=www.bbc.com |language=en}}</ref>
* Changes in immune function during winter - one possibility is a reduction in vitamin D, and another is the effect of cold on mucous membranes in the nose.<ref>{{Cite web |last=Kashef |first=Ziba |date=2019-05-13 |title=Flu virus' best friend: low humidity |url=https://news.yale.edu/2019/05/13/flu-virus-best-friend-low-humidity |access-date=2023-09-13 |website=YaleNews |language=en}}</ref><ref>{{Cite web |last=LaMotte |first=Sandee |date=6 December 2022 |title=Scientists finally know why people get more colds and flu in winter |url=https://www.cnn.com/2022/12/06/health/why-winter-colds-flu-wellness/index.html |access-date=2023-09-14 |website=CNN |language=en}}</ref>
* Abundance of vectors such as mosquitoes.<ref>{{Cite book |last1=Medicine |first1=Institute of |url=https://books.google.com/books?id=lDClwYeJdCEC |title=Vector-Borne Diseases: Understanding the Environmental, Human Health, and Ecological Connections: Workshop Summary |last2=Health |first2=Board on Global |last3=Threats |first3=Forum on Microbial |date=2008-03-18 |publisher=National Academies Press |isbn=978-0-309-17770-2 |language=en}}</ref>

=== Human behaviour ===
[[File:Punch-A_Court_for_King_Cholera.png|thumb|''A Court for King Cholera.'' Illustration from ''[[Punch (magazine)|Punch]]'' (1852).]]
Changes in behaviour can affect the likelihood or severity of epidemics. The classic example is the [[1854 Broad Street cholera outbreak]], in which a cholera outbreak was mitigated by removing a supply of contaminated water - an event now regarded as the foundation of the science of [[epidemiology]].<ref>{{Cite journal |last=Tulchinsky |first=Theodore H. |date=2018 |title=John Snow, Cholera, the Broad Street Pump; Waterborne Diseases Then and Now |journal=Case Studies in Public Health |pages=77–99 |doi=10.1016/B978-0-12-804571-8.00017-2 |pmc=7150208|isbn=9780128045718 }}</ref> Urbanisation and overcrowding (e.g. in [[refugee camp]]s) increase the likelihood of disease outbreaks.<ref>{{Cite journal |last=Neiderud |first=Carl-Johan |date=2015-06-24 |title=How urbanization affects the epidemiology of emerging infectious diseases |journal=Infection Ecology & Epidemiology |volume=5 |issue=1 |pages=10.3402/iee.v5.27060 |doi=10.3402/iee.v5.27060 |issn=2000-8686 |pmc=4481042 |pmid=26112265|bibcode=2015InfEE...527060N }}</ref><ref>{{Cite journal |last1=Altare |first1=Chiara |last2=Kahi |first2=Vincent |last3=Ngwa |first3=Moise |last4=Goldsmith |first4=Amelia |last5=Hering |first5=Heiko |last6=Burton |first6=Ann |last7=Spiegel |first7=Paul |date=1 September 2019 |title=Infectious disease epidemics in refugee camps: a retrospective analysis of UNHCR data (2009-2017) |url=https://www.joghr.org/article/12009-infectious-disease-epidemics-in-refugee-camps-a-retrospective-analysis-of-unhcr-data-2009-2017 |journal=Journal of Global Health Reports |language=en |volume=3 |pages=e2019064 |doi=10.29392/joghr.3.e2019064|s2cid=207998081 |doi-access=free }}</ref> A factor which contributed to the initial rapid increase in the [[Western African Ebola virus epidemic|2014 Ebola virus epidemic]] was [[Bathing the dead|ritual bathing]] of (infective) corpses; one of the control measures was an education campaign to change behaviour around funeral rites.<ref>{{Cite web |last=Maxmen |first=Amy | author-link=Amy Maxmen|date=30 January 2015 |title=How the Fight Against Ebola Tested a Culture's Traditions |url=https://www.nationalgeographic.com/adventure/article/150130-ebola-virus-outbreak-epidemic-sierra-leone-funerals-1 |archive-url=https://web.archive.org/web/20210308231136/https://www.nationalgeographic.com/adventure/article/150130-ebola-virus-outbreak-epidemic-sierra-leone-funerals-1 |url-status=dead |archive-date=March 8, 2021 |access-date=14 September 2023 |website=National Geographic |language=en}}</ref>

=== Changes in the host population ===
The level of immunity to a disease in a population - [[herd immunity]] - is at its peak after a disease outbreak or a vaccination campaign. In the following years, immunity will decline, both within individuals and in the population as a whole as older individuals die and new individuals are born. Eventually, unless there is another vaccination campaign, an outbreak or epidemic will recur.<ref>{{Cite journal |last1=Yang |first1=Luojun |last2=Grenfell |first2=Bryan T |last3=Mina |first3=Michael J |date=February 2020 |title=Waning immunity and re-emergence of measles and mumps in the vaccine era |url=https://linkinghub.elsevier.com/retrieve/pii/S1879625720300304 |journal=Current Opinion in Virology |language=en |volume=40 |pages=48–54 |doi=10.1016/j.coviro.2020.05.009|pmid=32634672 |s2cid=220414525 }}</ref>

It's also possible for disease which is endemic in one population to become epidemic if it is introduced into a novel setting where the host population is not immune. An example of this was the introduction European diseases such as smallpox into indigenous populations during the 16th century.<ref>{{cite web |title=Stacy Goodling, "Effects of European Diseases on the Inhabitants of the New World" |url=http://www.millersville.edu/~columbus/papers/goodling.html |url-status=dead |archive-url=https://web.archive.org/web/20080510163413/http://www.millersville.edu/~columbus/papers/goodling.html |archive-date=10 May 2008}}</ref>

=== Zoonosis ===
[[File:Figure 3- Examples of Zoonotic Diseases and Their Affected Populations (6323431516).jpg|thumb|Possibilities for zoonotic disease transmissions]]
A [[zoonosis]] is an [[infectious disease]] of humans caused by a pathogen that can [[Cross-species transmission|jump]] from a non-human host to a human.<ref>{{Cite web |date=29 July 2020 |title=Zoonoses |url=https://www.who.int/news-room/fact-sheets/detail/zoonoses |access-date=14 September 2023 |website=World Health Organization |language=en}}</ref> Major diseases such as [[Ebola virus disease]] and [[salmonellosis]] are zoonoses. [[HIV]] was a zoonotic disease transmitted to humans in the early part of the 20th century, though it has now evolved into a separate human-only disease.<ref name="Orgin2011">{{cite journal |vauthors=Sharp PM, Hahn BH |date=September 2011 |title=Origins of HIV and the AIDS pandemic |journal=Cold Spring Harbor Perspectives in Medicine |volume=1 |issue=1 |pages=a006841 |doi=10.1101/cshperspect.a006841 |pmc=3234451 |pmid=22229120}}</ref> Some strains of [[Influenza A virus subtype H5N1|bird flu]] and [[Influenza A virus subtype H1N1|swine flu]] are zoonoses; these viruses occasionally recombine with human strains of the flu and can cause [[pandemic]]s such as the [[1918 Spanish flu]] or the [[2009 swine flu]].<ref>{{cite journal |vauthors=Scotch M, Brownstein JS, Vegso S, Galusha D, Rabinowitz P |date=September 2011 |title=Human vs. animal outbreaks of the 2009 swine-origin H1N1 influenza A epidemic |journal=EcoHealth |volume=8 |issue=3 |pages=376–380 |doi=10.1007/s10393-011-0706-x |pmc=3246131 |pmid=21912985}}</ref>

==Types==
{{Expand section|fuller description, examples, less jargon|date=September 2023}}

=== Common source outbreak ===
In a common source outbreak epidemic, the affected individuals had an exposure to a common agent. If the exposure is singular and all of the affected individuals develop the disease over a single exposure and incubation course, it can be termed as a point source outbreak. If the exposure was continuous or variable, it can be termed as a continuous outbreak or intermittent outbreak, respectively.<ref name="Principles of Epidemiology"/>{{rp|56}}

===Propagated outbreak===
In a propagated outbreak, the disease spreads person-to-person. Affected individuals may become independent reservoirs leading to further exposures.<ref name="Principles of Epidemiology"/>{{rp|56}}Many epidemics will have characteristics of both common source and propagated outbreaks (sometimes referred to as '''mixed outbreak''').{{citation needed|date=February 2023}}

For example, secondary person-to-person spread may occur after a common source exposure or an environmental vector may spread a [[zoonotic]] diseases agent.<ref name="Principles of Epidemiology"/>{{rp|56–58}}

==Preparation==
{{Update|part=section|date=September 2023|reason=Needing update to reflect changes in theory and practice since the COVID-19 pandemic}}
Preparations for an epidemic include having a disease surveillance system; the ability to quickly dispatch emergency workers, especially local-based emergency workers; and a legitimate way to guarantee the safety and health of health workers.<ref name=gerson>{{cite news | vauthors = Gerson M | url = https://www.washingtonpost.com/opinions/the-next-epidemic/2015/03/26/aee9f344-d3e8-11e4-a62f-ee745911a4ff_story.html | title = The next epidemic | newspaper = [[The Washington Post]] | date = 26 March 2015 }}</ref><ref>{{cite journal | vauthors = Gates B | title = The next epidemic--lessons from Ebola | journal = The New England Journal of Medicine | volume = 372 | issue = 15 | pages = 1381–4 | date = April 2015 | pmid = 25853741 | doi = 10.1056/NEJMp1502918 | author-link = Bill Gates | doi-access = free }}</ref>

Effective preparations for a response to a pandemic are multi-layered. The first layer is a disease surveillance system. [[Tanzania]], for example, runs a national lab that runs testing for 200 health sites and tracks the spread of infectious diseases. The next layer is the actual response to an emergency. According to U.S.-based columnist Michael Gerson in 2015, only the U.S. military and [[NATO]] have the global capability to respond to such an emergency.<ref name=gerson /> Still, despite the most extensive preparatory measures, a fast-spreading pandemic may easily exceed and overwhelm existing health-care resources.<ref name="ACAIM-WACEM COVID-19 Consensus Paper">{{cite journal |display-authors=6 |vauthors=Stawicki SP, Jeanmonod R, Miller AC, Paladino L, Gaieski DF, Yaffee AQ, De Wulf A, Grover J, Papadimos TJ, Bloem C, Galwankar SC, Chauhan V, Firstenberg MS, Di Somma S, Jeanmonod D, Garg SM, Tucci V, Anderson HL, Fatimah L, Worlton TJ, Dubhashi SP, Glaze KS, Sinha S, Opara IN, Yellapu V, Kelkar D, El-Menyar A, Krishnan V, Venkataramanaiah S, Leyfman Y, Saoud Al Thani HA, Wb Nanayakkara P, Nanda S, Cioè-Peña E, Sardesai I, Chandra S, Munasinghe A, Dutta V, Dal Ponte ST, Izurieta R, Asensio JA, Garg M |year=2020 |title=The 2019-2020 Novel Coronavirus (Severe Acute Respiratory Syndrome Coronavirus 2) Pandemic: A Joint American College of Academic International Medicine-World Academic Council of Emergency Medicine Multidisciplinary COVID-19 Working Group Consensus Paper |url=https://www.researchgate.net/publication/340903626 |journal=Journal of Global Infectious Diseases |volume=12 |issue=2 |pages=47–93 |doi=10.4103/jgid.jgid_86_20 |pmc=7384689 |pmid=32773996 |s2cid=218754925 |doi-access=free}}</ref> Consequently, early and aggressive mitigation efforts, aimed at the so-called "epidemic curve flattening" need to be taken.<ref name="ACAIM-WACEM COVID-19 Consensus Paper"/> Such measures usually consist on non-pharmacological interventions such as social/physical distancing, aggressive contact tracing, "stay-at-home" orders, as well as appropriate personal protective equipment (i.e., masks, gloves, and other physical barriers to spread).<ref name="ACAIM-WACEM COVID-19 Consensus Paper"/>

Moreover, India has taken significant strides in its efforts to prepare for future respiratory pandemics through the development of the National Pandemic Preparedness Plan for Respiratory Viruses using a multisectoral approach.<ref>{{Cite web |title=India gets "PRET" for future pandemics: a national consultation on developing the content framework for the National Pandemic Preparedness Plan for Respiratory Viruses |url=https://www.who.int/news/item/12-02-2024-india-gets-pret--for-future-pandemics-a-national-consultation-on-developing-the-content-framework-for-the-national-pandemic-preparedness-plan-for-respiratory-viruses |access-date=2025-03-13 |website=www.who.int |language=en}}</ref>

Preceding this national effort, a regional workshop on the Preparedness and Resilience for Emerging Threats (PRET) initiative was organized by WHO's South-East Asia Regional Office on October 12–13, 2023. Recognizing that the same capacities and capabilities can be leveraged and applied for groups of pathogens based on their mode of transmission, the workshop aimed to facilitate pandemic planning efficiency for countries in the region. The participating countries, in the aftermath of the workshop, outlined their immediate next steps and sought support from WHO and its partners to bolster regional preparedness against respiratory pathogen pandemics.<ref>{{Cite web |title="PRET" for future pandemics? Countries in WHO’s South-East Asia Region outline steps to update respiratory pandemic plans |url=https://www.who.int/news-room/photo-story/detail/PRET-for-respiratory-pandemics-countries-in-who-south-easta-asia-region-outline-steps-to-update-pandemic-plans |access-date=2025-03-13 |website=www.who.int |language=en}}</ref>

== See also ==
{{Portal|Pandemics|Medicine}}
{{div col |colwidth=25em |small=no}}
* [[List of epidemics]]
* [[Epidemiology]]
* [[Endemic (epidemiology)]]
* [[Pandemic]]
* [[Syndemic]]
* [[European Centre for Disease Prevention and Control]]
* [[Centers for Disease Control and Prevention]]
* [[Mathematical modelling of infectious disease]]
* [[Compartmental models in epidemiology|Epidemic model]]
* [[Biosecurity]]
* [[Pathogen transmission]]
{{div col end}}

== References ==
{{reflist}}

== Further reading ==
{{refbegin}}
* Brook, Timothy; et al. "Comparative pandemics: the Tudor–Stuart and Wanli–Chongzhen years of pestilence, 1567–1666" ''Journal of Global History'' (2020) 14#3 pp 363–379 emphasis on Chinese history, compared to England
* Eisenberg, Merle, and Lee Mordechai. "The Justinianic Plague and Global Pandemics: The Making of the Plague Concept." ''American Historical Review'' 125.5 (2020): 1632–1667.
* {{Cite news |last=Honigsbaum |first=Mark |date=18 October 2020 |title=How do pandemics end? In different ways, but it's never quick and never neat |language=en-GB |work=The Guardian |url=https://www.theguardian.com/commentisfree/2020/oct/18/how-do-pandemics-end-in-different-ways-but-its-never-quick-and-never-neat |access-date=28 October 2020 |issn=0261-3077}}
* {{cite journal |last=Lietaert Peerbolte |first=Bert Jan |date=September 2021 |title=The Book of Revelation: Plagues as Part of the Eschatological Human Condition |journal=[[Journal for the Study of the New Testament]] |publisher=[[SAGE Publications]] |volume=44 |issue=1 |pages=75–92 |doi=10.1177/0142064X211025496
|doi-access=free |issn=1745-5294 |s2cid=237332665 }}
* [[Maryn McKenna|McKenna, Maryn]], "Return of the Germs: For more than a century drugs and vaccines made astounding progress against infectious diseases. Now our best defenses may be social changes", ''[[Scientific American]]'', vol. 323, no. 3 (September 2020), pp.&nbsp;50–56. "What might prevent or lessen [the] possibility [of a virus emerging and finding a favorable human host] is more prosperity more equally distributed – enough that villagers in South Asia need not trap and sell bats to supplement their incomes and that, low-wage workers in the U.S. need not go to work while ill because they have no sick leave." (p.&nbsp;56.)
* {{cite web |url=https://ipbes.net/pandemics |title=Escaping the 'Era of Pandemics': Experts Warn Worse Crises to Come Options Offered to Reduce Risk |author=<!--Not stated-->|date=2020|publisher=[[Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services]]}}
{{refend}}

== External links ==
{{Wiktionary|epidemic}}
{{refbegin}}
* {{cite web | url = http://www.ecdc.europa.eu | title = European Centre for Disease Prevention and Control }}
* {{cite web | url = http://www.dundee.ac.uk/iea/ | title = International Epidemiological Association (IEA) | archive-url = https://web.archive.org/web/20101127143509/http://www.dundee.ac.uk/iea/ | archive-date = 2010-11-27 }}
* {{cite book | url = https://global.oup.com/academic/product/a-dictionary-of-epidemiology-9780199976720 | title = A Dictionary of Epidemiology (IEA) | date = 20 June 2014 | publisher = Oxford University Press | isbn = 978-0-19-997672-0 }}
* {{cite web | url = http://www.epidemiology.ch/history/betaversion.htm | title = People's Epidemiology Library | archive-url = https://web.archive.org/web/20120323082628/http://www.epidemiology.ch/history/betaversion.htm | archive-date = 2012-03-23 }}
* {{cite web | url = http://www.laprp.com/article.php?aid=348 | title = Video Discussion of the Prostate Cancer Epidemic }}
* {{cite web | url = http://vlab.infotech.monash.edu.au/simulations/cellular-automata/epidemic/ | work = Monash Virtual Laboratory | title = Simulations of epidemic spread across a landscape | archive-url = https://web.archive.org/web/20071104183725/http://vlab.infotech.monash.edu.au/simulations/cellular-automata/epidemic/ | archive-date = 2007-11-04 }}
{{refend}}

{{Concepts in infectious disease}}
{{Public health}}
{{Epidemics}}
{{Authority control}}

[[Category:Epidemics| ]]
[[Category:Biological hazards]]