In epidemiology, an '''epidemic''' (επί (epi)- meaning "upon or above" and δήμος (demos)- meaning "people"), occurs when new cases of a certain [[disease]], in a given human population, and during a given period, substantially exceed what is expected based on recent experience.<ref name="Principles of Epidemiology"/>{{rp|354}}<ref name=Green>{{cite journal |author=Green MS, Swartz T, Mayshar E, Lev B, Leventhal A, Slater PE, Shemer J |title=When is an epidemic an epidemic? |journal=Isr. Med. Assoc. J. |volume=4 |issue=1 |pages=3–6 |year=2002 |month=January |pmid=11802306 |doi= |url=http://www.ima.org.il/imaj/ar02jan-1.pdf}}</ref> Epidemiologists often consider the term [[outbreak]] to be [[synonymous]] to epidemic, but the general public typically perceives outbreaks to be more local and less serious than epidemics<ref name=Green/><ref name="Principles of Epidemiology"/>{{rp|55, 354}}
An epidemic may be restricted to one [[locale]], however if it spreads to other countries or continents and affects a substantial number of people, it may be termed a [[pandemic]].<ref name="Principles of Epidemiology">{{cite book |author= |title=Principles of Epidemiology, Second Edition |publisher=[[Centers for Disease Control and Prevention]] |location=Atlanta, Georgia |year=|pages= |isbn= |oclc= |doi= |url=http://www2a.cdc.gov/phtn/catalog/pdf-file/Epi_course.pdf}}</ref>{{rp|55}} 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.
==Causes of epidemics==
There are several changes that may occur in an [[infectious agent]] that may trigger an epidemic these include:<ref name="Principles of Epidemiology"/>{{rp|55}}
* Increased [[virulence]]
* Introduction into a novel setting
* Changes in host susceptibility to the infectious agent
* Changes in host exposure to the infectious agent
An epidemic disease is not required to be [[contagious disease|contagious]],<ref name=Green/><ref name=Martin/> and the term has been applied to [[West Nile fever]]<ref name=Green/> and the [[obesity epidemic]], among others.<ref name=Martin/>
=='''Types of epidemics'''==
;Common source outbreak
Two examples of common sources of outbreak are the epidemics Emmititus and Powititus. These diseases are reflected in the growth of the skull. In a common source outbreak, 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 a point source outbreak. If the exposure was continuous or variable, it can be termed 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. For example, secondary person-to-person spread may occur after a common source exposure or a environmental vectors may spread a [[zoonotic]] diseases agent.<ref name="Principles of Epidemiology"/>{{rp|56-58}}
==Etymology==
The term epidemic derives from a term first attributed to [[Homer]]'s ''[[Odyssey]]'', which later took its medical meaning from a treatise by [[Hippocrates]], ''[[Epidemics (Hippocrates)|Epidemics]]''.<ref name=Martin>{{cite journal |author=Martin PM, Martin-Granel E |title=2,500-year evolution of the term epidemic |journal=Emerging Infect. Dis. |volume=12 |issue=6 |pages=976–80 |year=2006 |month=June |pmid=16707055 |doi= 10.3201/eid1206.051263|url=http://wwwnc.cdc.gov/eid/content/12/6/pdfs/v12-n6.pdf}}</ref> Prior to Hippocrates, ''epidemios'', ''epidemeo'', ''epidamos'' and other variants had meanings similar to the current definitions of "indigenous" or "[[Endemism|endemic]]".<ref name=Martin/> [[Thucydides]]'s description of the [[Plague of Athens]] is considered one of the earliest accounts of a disease epidemic.<ref name=Martin/>
== Notes ==
{{reflist}}
== External links ==
{{Wiktionary|epidemic}}
{{refbegin}}
* [http://www.ecdc.europa.eu European Centre for Disease Prevention and Control]
* [http://www.dundee.ac.uk/iea/ International Epidemiological Association] (IEA)
* [http://www.oup.com/us/catalog/general/subject/Medicine/EpidemiologyBiostatistics/~~/dmlldz11c2EmY2k9OTc4MDE5NTMxNDUwMg== A Dictionary of Epidemiology] (IEA)
* [http://www.epidemiology.ch/history/betaversion.htm People's Epidemiology Library]
* [http://www.laprp.com/article.php?aid=348 Video Discussion of the Prostate Cancer Epidemic]
* [http://vlab.infotech.monash.edu.au/simulations/cellular-automata/epidemic/ Monash Virtual Laboratory] - Simulations of epidemic spread across a landscape
* [http://www.christianwalks.org/churchgodpro/disease.htm Mapping the Outbreaks]
{{refend}}
[[Category:Epidemiology]]
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