Epidemic

An epidemic (from

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.^{[1]: §1:72 [2]}

Any sudden increase in disease prevalence may generally be termed an epidemic. This may include

The term epidemic derives from a word form attributed to Homer's Odyssey, which later took its medical meaning from the Epidemics, a treatise by Hippocrates.^[5] Before Hippocrates, epidemios, epidemeo, epidamos, and other variants had meanings similar to the current definitions of "indigenous" or "endemic".^[5] Thucydides' description of the Plague of Athens is considered one of the earliest accounts of a disease epidemic.^[5] 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".^[6]

The term "epidemic" is often applied to diseases in non-human animals, although "epizootic" is technically preferable.^[7][8]

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:^{[9]: §1:72}

Antigenic Change

An antigen is a protein on the virus' surface that host antibodies can recognize and attack. Changes in the 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 mutations in the 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, 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.^[10] SARS-CoV2 has demonstrated antigenic drift and possibly shift as well.^[11]

Drug resistance

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:^[16] -

• airborne (as with influenza and COVID-19),
• fecal-oral (as with cholera and typhoid),
• vector-borne
  (malaria, Zika) and
• 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.^[17]

Another possibility, although rare, is that a pathogen may adapt to take advantage of a new mode of transmission^[18][19]

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,^[20][21] This may be due to one or more of the following underlying factors: -^[22]

• The ability of the pathogen to survive outside the host - e.g.
  water-borne cholera^[23] which becomes prevalent in tropical wet seasons, or influenza which peaks in temperate regions during winter.^[24][25]
• The behaviour of people susceptible to the disease - such as spending more time in close contact indoors.[26]
• Changes in immune function during winter - one possibility is a reduction in vitamin D, and another is the effect of cold on mucus membranes in the nose.^[27][28]
• Abundance of vectors such as mosquitoes.^[29]

Human behaviour

Changes in behaviour can affect the likelihood or severity of epidemics. The classic example is the

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.^[34]

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.^[35]

Zoonosis

A

Types

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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.[1]^: 56

Propagated outbreak

In a propagated outbreak, the disease spreads person-to-person. Affected individuals may become independent reservoirs leading to further exposures.^[1]: 56Many epidemics will have characteristics of both common source and propagated outbreaks (sometimes referred to as mixed outbreak).^{[citation needed]}

For example, secondary person-to-person spread may occur after a common source exposure or an environmental vector may spread a

^: 56–58

Preparation

This section needs to be updated. The reason given is: Needing update to reflect changes in theory and practice since the COVID-19 pandemic. Please help update this article to reflect recent events or newly available information. (September 2023)

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.[39]^[40]

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.^[39] Still, despite the most extensive preparatory measures, a fast-spreading pandemic may easily exceed and overwhelm existing health-care resources.^[41] Consequently, early and aggressive mitigation efforts, aimed at the so-called "epidemic curve flattening" need to be taken.^[41] 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).^[41]

See also

References

Further reading

• 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.
• Honigsbaum M (18 October 2020). "How do pandemics end? In different ways, but it's never quick and never neat". The Guardian.
  ISSN 0261-3077
  . Retrieved 28 October 2020.
• Lietaert Peerbolte BJ (September 2021). "The Book of Revelation: Plagues as Part of the Eschatological Human Condition".
  S2CID 237332665
  .
• 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. 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. 56.)
• "Escaping the 'Era of Pandemics': Experts Warn Worse Crises to Come Options Offered to Reduce Risk". Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services. 2020.

External links

Look up epidemic in Wiktionary, the free dictionary.

• "European Centre for Disease Prevention and Control".
• "International Epidemiological Association (IEA)". Archived from the original on 2010-11-27.
• A Dictionary of Epidemiology (IEA). Oxford University Press. 20 June 2014.
  ISBN 978-0-19-997672-0
  .
• "People's Epidemiology Library". Archived from the original on 2012-03-23.
• "Video Discussion of the Prostate Cancer Epidemic".
• "Simulations of epidemic spread across a landscape". Monash Virtual Laboratory. Archived from the original on 2007-11-04.