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Epidemic

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For other uses, see Epidemic (disambiguation).
Example of an epidemic showing the number of new infections over time.

An epidemic (from Greek ἐπί epi "upon or above" and δῆμος demos "people") is the rapid spread of disease to a large number of hosts in a given population within a short period of time. For example, in meningococcal infections, an attack rate in excess of 15 cases per 100,000 people for two consecutive weeks is considered an epidemic.[1][2]

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 to either an established pathogen or newly emerging novel pathogen is suddenly reduced below that found in the endemic equilibrium and the transmission threshold is exceeded.[3]

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 a pandemic.[1]: §1:72  The declaration of an epidemic usually requires a good understanding of a baseline rate of incidence; epidemics for certain diseases, such as influenza, are defined as reaching some defined increase in incidence above this baseline.[2] 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]

Definition

The Plague of Athens (c. 1652–1654) by Michiel Sweerts, illustrating the devastating epidemic that struck Athens in 430 BC, as described by the historian Thucydides

The 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 contagious disease (i.e. easily spread between persons) such as influenza; vector-borne diseases such as malaria; water-borne diseases such as cholera; and sexually transmitted diseases such as HIV/AIDS. The term can also be used for non-communicable health issues such as obesity.[2][4][5]

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]

Causes

There are several factors, in an infectious agent or its environment, that may contribute to causing an epidemic. These include:[1]: §1:72 

  • Increased virulence of the agent
  • Introduction 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.[7]
  • Increase in contagiousness, transmissibility or a change in the agent's mode of transmission
  • Changes in host susceptibility to the agent, such as waning herd immunity several years after a vaccination campaign.[8]
  • Zoonosis - an agent that jumps from a non-human host to a human[9]

Epidemics can be related to seasonality of certain infectious agents. Seasonality may enter into any of the eight key elements of the system: (1) susceptible recruitment via reproduction, (2) transmission, (3) acquired immunity and recovery, (4) waning immunity, (5) natural mortality, (6) symptomatology and pathology (which may be acute or chronic, depending on the disease), (7) disease-induced mortality, and (8) cross-species transmission.[10]  Influenza, the common cold, and other infections of the upper respiratory tract, such as sore throat, occur predominantly in the winter. There is another variation, both as regards the number of people affected and the number who die in successive epidemics: the severity of successive epidemics rises and falls over periods of five or ten years.[11]

Types

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 a point source outbreak. If the exposure was continuous or variable, it can be termed 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]: 56 Many 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 zoonotic diseases agent.[1]: 56–58 

Transmission

Main article: Pathogen transmission
  • Airborne transmission: Airborne transmission is the spread of infection by droplet nuclei or dust in the air. Without the intervention of winds or drafts the distance over which airborne infection takes place is short, say 10 to 20 feet.[12]
  • Arthropod transmission: Arthropod transmission takes place by an insect, either mechanically through a contaminated proboscis or feet, or biologically when there is growth or replication of an organism in the arthropod.[citation needed]
  • Biological transmission: Involving a normal biological process, e.g., passing a stage of development of the infecting agent in an intermediate host. Opposite to mechanical transmission.[citation needed]
  • Contact transmission: The disease agent is transferred directly by biting, sucking, chewing or indirectly by inhalation of droplets, drinking of contaminated water, traveling in contaminated vehicles.
  • Cyclopropagative transmission: The agent undergoes both development and multiplication in the transmitting vehicle.[citation needed]
  • Developmental transmission: The agent undergoes some development in the transmission vehicle.[citation needed]
  • Fecal-oral transmission: The infectious agent is shed by the infected host in feces and acquired by the susceptible host through the ingestion of contaminated material.
  • Horizontal transmission: Lateral spread to others in the same group and at the same time; spread to contemporaries.
  • Propagative transmission: The agent multiplies in the transmission vehicle.
  • Vertical transmission: From one generation to the next, perhaps transovarially or by intrauterine infection of the fetus. Some retroviruses are transmitted in the germline, i.e. their genetic material is integrated into the DNA of either the ovum or sperm.[13]

Preparation

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.[14][15]

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.[14] Still, despite the most extensive preparatory measures, a fast-spreading pandemic may easily exceed and overwhelm existing health-care resources.[12] Consequently, early and aggressive mitigation efforts, aimed at the so-called "epidemic curve flattening" need to be taken.[12] 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).[12]

See also

References

  1. ^ a b c d e f g Dicker, Richard C.; Coronado, Fátima; Koo, Denise; Parrish, R. Gibson (2012). Principles of epidemiology in public health practice; an introduction to applied epidemiology and biostatistics. 3rd ed (Third ed.). Atlanta, Georgia: Centers for Disease Control and Prevention.
  2. ^ a b c d Green MS, Swartz T, Mayshar E, Lev B, Leventhal A, Slater PE, Shemer J (January 2002). "When is an epidemic an epidemic?" (PDF). The Israel Medical Association Journal. 4 (1): 3–6. PMID 11802306.
  3. ^ Callow PP, ed. (1998). "Epidemic". The Encyclopedia of Ecology and Environmental Management. Oxford: Blackwell Science Ltd. p. 246. ISBN 0-86542-838-7.
  4. ^ Controlling the global obesity epidemic, the World Health Organization
  5. ^ a b c d Martin PM, Martin-Granel E (June 2006). "2,500-year evolution of the term epidemic". Emerging Infectious Diseases. 12 (6): 976–80. doi:10.3201/eid1206.051263. PMC 3373038. PMID 16707055.
  6. ^ Lodge, Thomas (1603). 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. London: Edward White. CHAP. 1. Of the nature and essence of the Plague
  7. ^ "Stacy Goodling, "Effects of European Diseases on the Inhabitants of the New World"". Archived from the original on 10 May 2008.
  8. ^ Yang, Luojun; Grenfell, Bryan T; Mina, Michael J (February 2020). "Waning immunity and re-emergence of measles and mumps in the vaccine era". Current Opinion in Virology. 40: 48–54. doi:10.1016/j.coviro.2020.05.009.
  9. ^ "Zoonoses". World Health Organization. 29 July 2020. Retrieved 5 September 2023.
  10. ^ Martinez ME (November 2018). "The calendar of epidemics: Seasonal cycles of infectious diseases". PLOS Pathogens. 14 (11): e1007327. doi:10.1371/journal.ppat.1007327. PMC 6224126. PMID 30408114.
  11. ^ Marcovitch H, ed. (2009). "Epidemic". Black's Medical Dictionary (42nd ed.). London: A&C Black. ISBN 978-1-4081-4564-7.
  12. ^ a b c d Stawicki SP, Jeanmonod R, Miller AC, Paladino L, Gaieski DF, Yaffee AQ, et al. (2020). "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". Journal of Global Infectious Diseases. 12 (2): 47–93. doi:10.4103/jgid.jgid_86_20. PMC 7384689. PMID 32773996. S2CID 218754925.
  13. ^ Studdert VP, Gay CC, Charles Blood DC, eds. (2012). "Transmission". Saunders Comprehensive Veterinary Dictionary (4th ed.). Philadelphia: Elsevier Health Sciences. ISBN 978-0-7020-3231-8.
  14. ^ a b Gerson M (26 March 2015). "The next epidemic". The Washington Post.
  15. ^ Gates B (April 2015). "The next epidemic--lessons from Ebola". The New England Journal of Medicine. 372 (15): 1381–4. doi:10.1056/NEJMp1502918. PMID 25853741.

Further reading

External videos
video icon Presentation by Brown on Influenza, March 5, 2019, C-SPAN
Look up epidemic in Wiktionary, the free dictionary.
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