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 infectious disease to a large number of people in a given population within a short period of time, usually two weeks or less. 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]

Ancient Greek Version of the Greek language used from roughly the 9th century BCE to the 6th century CE

The Ancient Greek language includes the forms of Greek used in Ancient Greece and the ancient world from around the 9th century BCE to the 6th century CE. It is often roughly divided into the Archaic period, Classical period, and Hellenistic period. It is antedated in the second millennium BCE by Mycenaean Greek and succeeded by medieval Greek.

In epidemiology, the attack rate is the biostatistical measure of frequency of morbidity, or speed of spread, in an at risk population. It is used in hypothetical predictions and during actual outbreaks of disease. An at risk population is defined as one that has no immunity to the attacking pathogen which can be either a novel pathogen or an established pathogen. It is used to project the number of victims to expect during an epidemic. This aids in marshalling resources for delivery of medical care as well as production of vaccines and/or anti-viral and anti-bacterial medicines. The rate is arrived at by taking the number of new cases in the population at risk and dividing by the number of persons at risk in the population.

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.

Pandemic global epidemic of infectious disease

A pandemic is an epidemic of disease that has spread across a large region; for instance multiple continents, or even worldwide. A widespread endemic disease that is stable in terms of how many people are getting sick from it is not a pandemic. Further, flu pandemics generally exclude recurrences of seasonal flu. Throughout history, there have been a number of pandemics, such as smallpox and tuberculosis. One of the most devastating pandemics was the Black Death, which killed over 75 million people in 1350. The most recent pandemics include the HIV pandemic as well as the 1918 and 2009 H1N1 pandemics.

A baseline in medicine is information found at the beginning of a study or other initial known value which is used for comparison with later data. The concept of a baseline is essential to the daily practice of medicine in order to establish a relative rather than absolute meaning to data. The meaning of baseline in medicine is very similar to that of the running baseline (baseball) being the direct path that a baserunner is taking to the base he is in route to. If the baserunner is outside the 3 foot margin around his running baseline, then he is considered out. Whereas if a patient with kidney failure suddenly has a creatinine of 5.0 mg/dL, then his creatinine is out of his normal. For that person with kidney failure, absolute normal no longer applies because he will never again be able to obtain an absolutely normal creatinine level with kidneys that no longer function properly.

Incidence (epidemiology) measure of the probability of occurrence of a given medical condition in a population within a specified period of time

Incidence in epidemiology is a measure of the probability of occurrence of a given medical condition in a population within a specified period of time. Although sometimes loosely expressed simply as the number of new cases during some time period, it is better expressed as a proportion or a rate with a denominator.

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 an emotionally-charged one. [6]

Homer name ascribed by the ancient Greeks to the legendary author of the Iliad and the Odyssey

Homer is the legendary author of the Iliad and the Odyssey, two epic poems that are the central works of ancient Greek literature. The Iliad is set during the Trojan War, the ten-year siege of the city of Troy by a coalition of Greek kingdoms. It focuses on a quarrel between King Agamemnon and the warrior Achilles lasting a few weeks during the last year of the war. The Odyssey focuses on the ten-year journey home of Odysseus, king of Ithaca, after the fall of Troy. Many accounts of Homer's life circulated in classical antiquity, the most widespread being that he was a blind bard from Ionia, a region of central coastal Anatolia in present-day Turkey. Modern scholars consider these accounts legendary.

<i>Odyssey</i> Epic poem attributed to Homer

The Odyssey is one of two major ancient Greek epic poems attributed to Homer. It is, in part, a sequel to the Iliad, the other Homeric epic. The Odyssey is fundamental to the modern Western canon; it is the second-oldest extant work of Western literature, while the Iliad is the oldest. Scholars believe the Odyssey was composed near the end of the 8th century BC, somewhere in Ionia, the Greek coastal region of Anatolia.

Hippocrates ancient Greek physician

Hippocrates of Kos, also known as Hippocrates II, was a Greek physician of the Age of Pericles, who is considered one of the most outstanding figures in the history of medicine. He is often referred to as the "Father of Medicine" in recognition of his lasting contributions to the field as the founder of the Hippocratic School of Medicine. This intellectual school revolutionized medicine in ancient Greece, establishing it as a discipline distinct from other fields with which it had traditionally been associated, thus establishing medicine as a profession.

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. [6]

The Chambers Dictionary (TCD) was first published by William and Robert Chambers as Chambers's English Dictionary in 1872. It was an expanded version of Chambers's Etymological Dictionary of 1867, compiled by James Donald. A second edition came out in 1898, and was followed in 1901 by a new compact edition called Chambers's Twentieth Century Dictionary.

Stedman's Medical Dictionary is designed in a dictionary format to provide the language of medicine, nursing and the allied health profession. It is made up of terms, images and appendices. It can be used to look up medical terms, abbreviation, acronyms, measurements and more. Pronunciation and word etymology are also provided with most definitions. There are over 54,000 terms and 900 illustrations included in the current print edition of Stedman's Medical Dictionary. If a hard copy of the text is purchased, it comes with a CD that can be easily searched and navigated. The CD also includes audio pronunciation to all terms and even more images. Most of Stedman’s products can now be purchased in electronic only versions. These come with all the terms, definitions, images, appendixes with the benefits of electronic searching and networking.

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 Organisation [7] ), 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. Certain epidemics occur at certain seasons.

For example, whooping-cough occurs in spring, whereas measles produces two epidemics, one in winter and one in March. 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 as mixed outbreak).

For example, secondary person-to-person spread may occur after a common source exposure or an environmental vectors may spread a zoonotic diseases agent. [1] :56–58

Transmission

[9]

Response

Experts[ who? ] suggest that the best way to prepare for an epidemic is to have a disease surveillance system, be able to quickly dispatch emergency workers, especially local-based emergency workers, and have a legitimate way to guarantee the safety and health of health workers. [10]

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, only the U.S. military and NATO have the global capability to respond to such an emergency. [10]

Gates proposed that the world responded slowly to the Ebola virus outbreak because of a lack of preparation. Two weeks after the 2013 typhoon hit the Philippines, over 150 foreign medical teams were on the ground helping with injured victims. After the 2005 earthquake in Pakistan, a team of aid workers who had been helping Afghan refugees for several decades were able to get to the victims in less than 24 hours. Dr. Bruce Aylward, assistant director general for emergencies at the World Health Organization, says that in the case of the Ebola outbreak, "there was no way anyone could guarantee the right of medical evacuation for people affected by Ebola.”

The CDC will scale back global disease prevention efforts by 80 percent by October 2019 due to a lack of funding (as funding had been temporarily increased in 2014 to address the Ebola epidemic). [11]

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.

Zoonosis infectious disease that is transmitted between species (sometimes by a vector) from animals other than humans to humans or from humans to other animals

Zoonoses are infectious diseases caused by bacteria, viruses and parasites that spread between animals and humans.

Infection invasion of a host by disease-causing organisms

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. Infectious disease, also known as transmissible disease or communicable disease, is illness resulting from an infection.

Shigellosis Human disease

Shigellosis is an infection of the intestines caused by Shigella bacteria. Symptoms generally start one to two days after exposure and include diarrhea, fever, abdominal pain, and feeling the need to pass stools even when the bowels are empty. The diarrhea may be bloody. Symptoms typically last five to seven days. Complications can include reactive arthritis, sepsis, seizures, and hemolytic uremic syndrome.

Tularemia primary bacterial infectious disease that has material basis in Francisella tularensis, which is transmitted by dog tick bite (Dermacentor variabilis), transmitted by deer flies (Chrysops sp) or transmitted by contact with infected animal tissues.

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.

In epidemiology, an outbreak is a sudden increase in occurrences of a disease in a particular time and place. It may affect a small and localized group or impact upon thousands of people across an entire continent. Two linked cases of a rare infectious disease may be sufficient to constitute an outbreak. 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.

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.

In epidemiology, the basic reproduction number of an infection can be thought of as the number of cases one case generates on average over the course of its infectious period, in an otherwise uninfected population.

In epidemiology, an infection is said to be endemic in a population when that infection is constantly maintained at a baseline level in a geographic area without external inputs. For example, chickenpox is endemic in the UK, but malaria is not. Every year, there are a few cases of malaria reported in the UK, but these do not lead to sustained transmission in the population due to the lack of a suitable vector. While it might be common to say that AIDS is "endemic" in Africa, meaning found in an area, this is a use of the word in its etymological, rather than epidemiological, form. AIDS cases in Africa are increasing, so the disease is not in an endemic steady state. It is correct to call the spread of AIDS in Africa an epidemic.

Mathematical models can project how infectious diseases progress to show the likely outcome of an epidemic and help inform public health interventions. Models use some basic assumptions and 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 in deciding which intervention/s to avoid and which to trial.

Natural reservoir long-term host of the pathogen of an infectious disease

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 control is the discipline concerned with preventing nosocomial or healthcare-associated infection, a practical sub-discipline of epidemiology. It is an essential, though often underrecognized and undersupported, 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. Anti-infective agents include antibiotics, antibacterials, antifungals, antivirals and antiprotozoals.

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.

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

In health care facilities, isolation represents one of several measures that can be taken to implement infection control: the prevention of contagious diseases from being spread 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 others. 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".

Social distancing

Social distancing is a term applied to certain nonpharmaceutical infection control actions that are taken by public health officials to stop or slow down the spread of a highly contagious disease. The objective of social distancing is to reduce the probability of contact between persons carrying an infection, and others who are not infected, so as to minimize disease transmission, morbidity and ultimately, mortality.

Airborne disease disease that is caused by pathogens and transmitted through the air

An airborne disease is any disease that is caused by pathogens that can be transmitted through the air. Such diseases include many of considerable importance both in human and veterinary medicine. The relevant pathogens may be viruses, bacteria, or fungi, and they may be spread through breathing, talking, coughing, sneezing, raising of dust, spraying of liquids, toilet flushing or any activities which generates aerosol particles or droplets. Human airborne diseases do not include conditions caused by air pollution such as Volatile Organic Compounds (VOCs), gases and any airborne particles, though their study and prevention may help inform the science of airborne disease transmission.

Transmission-based precautions are additional infection-control precautions in health care, and 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. The latter require additional control measures to effectively prevent transmission.

Super-spreader

A super-spreader is a host—an organism infected with a disease—that disproportionally infects more secondary contacts than other hosts who are also infected with the same disease. A sick human can be a super-spreader; they would be more likely to infect others than most people with the disease. Super-spreaders are thus of high concern in epidemiology.

Spillover infection, also known as pathogen spillover and spillover event, occurs when a reservoir population with a high pathogen prevalence comes into contact with a novel host population. The infection is transmitted from the reservoir population and may or may not be transmitted within the host population.

Ebola virus disease Viral hemorrhagic fever of humans and other primates caused by ebolaviruses

Ebola virus disease (EVD), also known as Ebola hemorrhagic fever (EHF) or simply Ebola, is a viral hemorrhagic fever of humans and other primates caused by ebolaviruses. Signs and symptoms typically start between two days and three weeks after contracting the virus with a fever, sore throat, muscular pain, and headaches. Vomiting, diarrhea and rash usually follow, along with decreased function of the liver and kidneys. At this time, some people begin to bleed both internally and externally. The disease has a high risk of death, killing between 25 and 90 percent of those infected, with an average of about 50 percent. This is often due to low blood pressure from fluid loss, and typically follows six to sixteen days after symptoms appear.

References

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  2. 1 2 3 4 Green MS; Swartz T; Mayshar E; Lev B; Leventhal A; Slater PE; Shemer Js (January 2002). "When is an epidemic an epidemic?". Isr. Med. Assoc. J. 4 (1): 3–6. PMID   11802306.
  3. "epidemic". The Encyclopedia of Ecology and Environmental Management, Blackwell Science. Oxford: Blackwell Publishers, 1998. Credo Reference. Web. 17 September 2012.
  4. 1 2 3 4 5 Martin PM, Martin-Granel E (June 2006). "2,500-year evolution of the term epidemic" (PDF). Emerging Infect. Dis. 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. 1 2 "When is an epidemic an epidemic?" (PDF). Israel Medical Association Journal. 4 (1): 3–6. 2002. PMID   11802306.Unknown parameter |vauhors= ignored (help)
  7. Controlling the global obesity epidemic, the World Health Organisation
  8. "Epidemic." Black's Medical Dictionary, 42nd Edition. London: A&C Black, 201e. Web. 17 September 2012.
  9. "transmission." Saunders Comprehensive Veterinary Dictionary. Philadelphia: Elsevier Health Sciences, 2007. Credo Reference. Web. 17 September 2012.
  10. 1 2 Gerson, Michael (30 March 2015). "The next epidemic". The Greenville News. South Carolina, US.
  11. Thomsen, Jacqueline (2 February 2018). "CDC to cut global disease prevention efforts by 80 percent".