Epidemic

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Example of an epidemic showing the number of new infections over time. 2014 West Africa Ebola Epidemic - New Cases per Week.svg
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 people 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]

Contents

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] 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.

Definition

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. [4] Before Hippocrates, epidemios, epidemeo, epidamos, and other variants had meanings similar to the current definitions of "indigenous" or "endemic". [4] Thucydides' description of the Plague of Athens is considered one of the earliest accounts of a disease epidemic. [4] By the early 17th century, the terms endemic and epidemic referred to contrasting conditions of population-level disease, with the endemic condition at low rates of occurrence and the epidemic condition widespread. [5] The term "epidemic" has become emotionally charged. [2]

The Centers for Disease Control and Prevention defines epidemic broadly: "the occurrence of more cases of disease, injury, or other health condition than expected in a given area or among a specific group of persons during a particular period. Usually, the cases are presumed to have a common cause or to be related to one another in some way (see also outbreak)." [1] The terms "epidemic" and "outbreak" have often been used interchangeably. Researchers Manfred S. Green and colleagues propose that the latter term be restricted to smaller events, pointing out that Chambers Concise Dictionary and Stedman's Medical Dictionary acknowledge this distinction. [2]

Causes

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 Plague in an Ancient City LACMA AC1997.10.1 (1 of 2).jpg
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

There are several changes that may occur in an infectious agent that may trigger an epidemic. These include: [1] :55

An epidemic disease is not required to be contagious, [2] [4] and the term has been applied to West Nile fever [2] and the obesity epidemic (e.g., by the World Health Organization [6] ), among others. [4]

The conditions which govern the outbreak of epidemics include infected food supplies such as contaminated drinking water and the migration of populations of certain animals, such as rats or mosquitoes, which can act as disease vectors.

Epidemics can be related to seasonality of certain infectious. 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. [7]  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. [8]

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).

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

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. [11] [12]

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. [11] Still, despite the most extensive preparatory measures, a fast-spreading pandemic may easily exceed and overwhelm existing health-care resources. [9] Consequently, early and aggressive mitigation efforts, aimed at the so-called "epidemic curve flattening" need to be taken. [9] 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). [9]

See also

Related Research Articles

Cholera Bacterial infection of the small intestine

Cholera is an infection of the small intestine by some strains of the bacterium Vibrio cholerae. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea that lasts a few days. Vomiting and muscle cramps may also occur. Diarrhea can be so severe that it leads within hours to severe dehydration and electrolyte imbalance. This may result in sunken eyes, cold skin, decreased skin elasticity, and wrinkling of the hands and feet. Dehydration can cause the skin to turn bluish. Symptoms start two hours to five days after exposure.

Pandemic Global epidemic of infectious disease

A pandemic is an epidemic of an infectious disease that has spread across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. A widespread endemic disease with a stable number of infected individuals is not a pandemic. Widespread endemic diseases with a stable number of infected individuals such as recurrences of seasonal influenza are generally excluded as they occur simultaneously in large regions of the globe rather than being spread worldwide.

Zoonosis Disease that can be transmitted from other species to humans

A zoonosis is an infectious disease caused by a pathogen that has jumped from an animal to a human. Typically, the first infected human transmits the infectious agent to at least one other human, who, in turn, infects others.

Infection Invasion of an organisms body tissues by disease-causing agents

An infection is the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to the infectious agents and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable disease, is an illness resulting from an infection.

Tularemia Infectious disease caused by the bacterium Francisella tularensis

Tularemia, also known as rabbit fever, is an infectious disease caused by the bacterium Francisella tularensis. Symptoms may include fever, skin ulcers, and enlarged lymph nodes. Occasionally, a form that results in pneumonia or a throat infection may occur.

Disease outbreak Sudden increase in occurrences of a disease

In epidemiology, an outbreak is a sudden increase in occurrences of a disease when cases are in excess of normal expectancy for the location or season. It may affect a small and localized group or impact upon thousands of people across an entire continent. The number of cases varies according to the disease-causing agent, and the size and type of previous and existing exposure to the agent. Outbreaks include epidemics, which term is normally only used for infectious diseases, as well as diseases with an environmental origin, such as a water or foodborne disease. They may affect a region in a country or a group of countries. Pandemics are near-global disease outbreaks when multiple countries across the world are infected.

In medicine, public health, and biology, transmission is the passing of a pathogen causing communicable disease from an infected host individual or group to a particular individual or group, regardless of whether the other individual was previously infected. The term strictly refers to the transmission of microorganisms directly from one individual to another by one or more of the following means:

Basic reproduction number Metric in epidemiology

In epidemiology, the basic reproduction number, or basic reproductive number, denoted , of an infection is the expected number of cases directly generated by one case in a population where all individuals are susceptible to infection. The definition assumes that no other individuals are infected or immunized. Some definitions, such as that of the Australian Department of Health, add the absence of "any deliberate intervention in disease transmission". The basic reproduction number is not necessarily the same as the effective reproduction number , which is the number of cases generated in the current state of a population, which does not have to be the uninfected state. is a dimensionless number and not a time rate, which would have units of time−1, or units of time like doubling time.

Mathematical models can project how infectious diseases progress to show the likely outcome of an epidemic and help inform public health interventions. Models use basic assumptions or collected statistics along with mathematics to find parameters for various infectious diseases and use those parameters to calculate the effects of different interventions, like mass vaccination programmes. The modelling can help decide which intervention(s) to avoid and which to trial, or can predict future growth patterns, etc.

Natural reservoir Living host, such as an animal or a plant, inside of which an infectious pathogen naturally lives and reproduces

In infectious disease ecology and epidemiology, a natural reservoir, also known as a disease reservoir or a reservoir of infection, is the population of organisms or the specific environment in which an infectious pathogen naturally lives and reproduces, or upon which the pathogen primarily depends for its survival. A reservoir is usually a living host of a certain species, such as an animal or a plant, inside of which a pathogen survives, often without causing disease for the reservoir itself. By some definitions a reservoir may also be an environment external to an organism, such as a volume of contaminated air or water.

Infection prevention and control is the discipline concerned with preventing healthcare-associated infections; a practical rather than academic sub-discipline of epidemiology. In Northern Europe, infection prevention and control is expanded from healthcare into a component in public health, known as "infection protection". It is an essential part of the infrastructure of health care. Infection control and hospital epidemiology are akin to public health practice, practiced within the confines of a particular health-care delivery system rather than directed at society as a whole.

An emergent virus is a virus that is either newly appeared, notably increasing in incidence/geographic range or has the potential to increase in the near future. Emergent viruses are a leading cause of emerging infectious diseases and raise public health challenges globally, given their potential to cause outbreaks of disease which can lead to epidemics and pandemics. As well as causing disease, emergent viruses can also have severe economic implications. Recent examples include the SARS-related coronaviruses, which have caused the 2002-2004 outbreak of SARS (SARS-CoV-1) and the 2019–21 pandemic of COVID-19 (SARS-CoV-2). Other examples include the human immunodeficiency virus which causes HIV/AIDS; the viruses responsible for Ebola; the H5N1 influenza virus responsible for avian flu; and H1N1/09, which caused the 2009 swine flu pandemic. Viral emergence in humans is often a consequence of zoonosis, which involves a cross-species jump of a viral disease into humans from other animals. As zoonotic viruses exist in animal reservoirs, they are much more difficult to eradicate and can therefore establish persistent infections in human populations.

Waterborne diseases diseases caused by pathogenic microorganisms transmitted in water

Waterborne diseases are conditions caused by pathogenic micro-organisms that are transmitted in water. These diseases can be spread while bathing, washing, drinking water, or by eating food exposed to contaminated water. While diarrhea and vomiting are the most commonly reported symptoms of waterborne illness, other symptoms can include skin, ear, respiratory, or eye problems. Waterborne diseases are impacted by a country's economy and also impact the economy by being costly to deal with.

Globalization, the flow of information, goods, capital, and people across political and geographic boundaries, allows infectious diseases to rapidly spread around the world, while also allowing the alleviation of factors such as hunger and poverty, which are key determinants of global health. The spread of diseases across wide geographic scales has increased through history. Early diseases that spread from Asia to Europe were bubonic plague, influenza of various types, and similar infectious diseases.

Bubonic plague Human and animal disease

Bubonic plague is one of three types of plague caused by the plague bacterium. One to seven days after exposure to the bacteria, flu-like symptoms develop. These symptoms include fever, headaches, and vomiting, as well as swollen and painful lymph nodes occurring in the area closest to where the bacteria entered the skin. Occasionally, the swollen lymph nodes, known as "buboes" may break open.

Isolation (health care) Measure taken to prevent contagious diseases from being spread

In health care facilities, isolation represents one of several measures that can be taken to implement in infection control: the prevention of communicable diseases from being transmitted from a patient to other patients, health care workers, and visitors, or from outsiders to a particular patient. Various forms of isolation exist, in some of which contact procedures are modified, and others in which the patient is kept away from all other people. In a system devised, and periodically revised, by the U.S. Centers for Disease Control and Prevention (CDC), various levels of patient isolation comprise application of one or more formally described "precaution".

Influenza Infectious disease, "the flu"

Influenza, commonly called "the flu", is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms typically begin 1–2 days and less typically 3-4 days after exposure to the virus and last for about 2–8 days. Diarrhea and vomiting can occur, particularly in children. Influenza may progress to pneumonia, which can be caused by the virus or by a subsequent bacterial infection. Other complications of infection include acute respiratory distress syndrome, meningitis, encephalitis, and worsening of pre-existing health problems such as asthma and cardiovascular disease.

Airborne transmission Disease transmission by airborne particles

Airborne or aerosol transmission is transmission of an infectious disease through small particles suspended in the air. Infectious diseases capable of airborne transmission include many of considerable importance both in human and veterinary medicine. The relevant infectious agent may be viruses, bacteria, or fungi, and they may be spread through breathing, talking, coughing, sneezing, raising of dust, spraying of liquids, flushing toilets, or any activities which generate aerosol particles or droplets.

Transmission-based precautions are infection-control precautions in health care, in addition to the so-called "standard precautions". They are the latest routine infection prevention and control practices applied for patients who are known or suspected to be infected or colonized with infectious agents, including certain epidemiologically important pathogens, which require additional control measures to effectively prevent transmission. Universal precautions are also important to address as far as transmission-based precautions. Universal precautions is the practice of treating all bodily fluids as if it is infected with HIV, HBV, or other blood borne pathogens.

In epidemiology, sporadic is a term used to refer to a disease which occurs only infrequently, haphazardly, irregularly or occasionally from time to time in a few isolated places with no discernible temporal or spatial pattern, as opposed to a recognizable epidemic or endemic pattern. The cases are so few and separated so widely in time and place that there exists little or no connection within them. They also do not show a recognizable common source of infection.

References

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  2. 1 2 3 4 5 6 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. 1 2 3 4 5 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.
  5. Lodge T (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.
  6. Controlling the global obesity epidemic, the World Health Organization
  7. 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.
  8. Marcovitch H, ed. (2009). "Epidemic". Black's Medical Dictionary (42nd ed.). London: A&C Black. ISBN   978-1-4081-4564-7.
  9. 1 2 3 4 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.
  10. 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.
  11. 1 2 Gerson M (26 March 2015). "The next epidemic". The Washington Post .
  12. 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 video
Nuvola apps kaboodle.svg Presentation by Brown on Influenza, March 5, 2019, C-SPAN