Example of an epidemic showing the number of new infections over time.
Anepidemic (fromGreek ἐπίepi "upon or above" and δῆμοςdemos "people") is the rapid spread ofdisease to a large number ofhosts in a given population within a short period of time. For example, inmeningococcal infections, anattack 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 hostimmunity to either an established pathogen or newly emergingnovel pathogen is suddenly reduced below that found in theendemic 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 as apandemic.[1]: §1:72 The declaration of an epidemic usually requires a good understanding of abaseline rate ofincidence; epidemics for certain diseases, such asinfluenza, are defined as reaching some defined increase in incidence above this baseline.[2] A few cases of a very raredisease may be classified as an epidemic, while many cases of a common disease (such as thecommon 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]
The United StatesCenters 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]
The termepidemic derives from a word form attributed toHomer'sOdyssey, which later took its medical meaning from theEpidemics, a treatise byHippocrates.[5] Before Hippocrates,epidemios,epidemeo,epidamos, and other variants had meanings similar to the current definitions of "indigenous" or "endemic".[5]Thucydides' description of thePlague of Athens is considered one of the earliest accounts of a disease epidemic.[5] By the early 17th century, the termsendemic andepidemic 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]
There are several factors that may contribute (individually or in combination) to causing an epidemic. There may be changes in apathogen, in the population that it can infect, in the environment, or in the interaction between all three. Factors include the following:[1]: §1:72
Illustration depicting the different mechanisms of antigenic shift and antigenic drift
Anantigen is aprotein on thevirus' surface that hostantibodies can recognize and attack. Changes in theantigenic 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 andantigenic shift.Antigenic drift arises over a period of time as an accumulation ofmutations in thevirus 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 avirus,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 isinfluenza.[9]SARS-CoV2 has demonstrated antigenic drift and possibly shift as well.[10]
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:[15] -
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.[16]
Another possibility, although rare, is that a pathogen may adapt to take advantage of a new mode of transmission[17][18]
Seasonal diseases arise due to the change in the environmental conditions, especially such as humidity and temperature, during different seasons. Many diseases displayseasonality,[19][20] This may be due to one or more of the following underlying factors: -[21]
The ability of the pathogen to survive outside the host - e.g.water-borne cholera[22] which becomes prevalent intropical wet seasons, or influenza which peaks in temperate regions during winter.[23][24]
The behaviour of people susceptible to the disease - such as spending more time in close contact indoors.[25]
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.[26][27]
A Court for King Cholera. Illustration fromPunch (1852).
Changes in behaviour can affect the likelihood or severity of epidemics. The classic example is the1854 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 ofepidemiology.[29] Urbanisation and overcrowding (e.g. inrefugee camps) increase the likelihood of disease outbreaks.[30][31] A factor which contributed to the initial rapid increase in the2014 Ebola virus epidemic wasritual bathing of (infective) corpses; one of the control measures was an education campaign to change behaviour around funeral rites.[32]
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.[33]
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.[34]
Azoonosis is aninfectious disease of humans caused by a pathogen that canjump from a non-human host to a human.[35] Major diseases such asEbola virus disease andsalmonellosis 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.[36] Some strains ofbird flu andswine flu are zoonoses; these viruses occasionally recombine with human strains of the flu and can causepandemics such as the1918 Spanish flu or the2009 swine flu.[37]
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
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 asmixed outbreak).[citation needed]
For example, secondary person-to-person spread may occur after a common source exposure or an environmental vector may spread azoonotic diseases agent.[1]: 56–58
This section needs to beupdated. 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.[38][39]
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 andNATO have the global capability to respond to such an emergency.[38] Still, despite the most extensive preparatory measures, a fast-spreading pandemic may easily exceed and overwhelm existing health-care resources.[40] Consequently, early and aggressive mitigation efforts, aimed at the so-called "epidemic curve flattening" need to be taken.[40] 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).[40]
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.[41]
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.[42]
^Dabour R, Meirson T, Samson AO (December 2016). "Global antibiotic resistance is mostly periodic".Journal of Global Antimicrobial Resistance.7:132–134.doi:10.1016/j.jgar.2016.09.003.PMID27788414.
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.
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.)
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