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In epidemiology
Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...

, an epidemic (επι (epi)- meaning "upon or above" and δεμος (demos)- meaning "people"), occurs when new cases of a certain disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...

, in a given human population, and during a given period, substantially exceed what is expected based on recent experience. Epidemiologists often consider the term outbreak
Outbreak is a term used in epidemiology to describe an occurrence of disease greater than would otherwise be expected at 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...

 to be synonymous to epidemic, but the general public typically perceives outbreaks to be more local and less serious than epidemics

An epidemic may be restricted to one locale
In computing, locale is a set of parameters that defines the user's language, country and any special variant preferences that the user wants to see in their user interface...

, however if it spreads to other countries or continents and affects a substantial number of people, it may be termed a pandemic
A pandemic is an epidemic of infectious disease that is spreading through human populations 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...

. The declaration of an epidemic usually requires a good understanding of a baseline
Baseline (medicine)
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...

 rate of incidence
Incidence (epidemiology)
Incidence is a measure of the risk of developing some new condition 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.Incidence proportion is the...

; epidemics for certain diseases, such as influenza
Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae , that affects birds and mammals...

, are defined as reaching some defined increase in incidence above this baseline. A few cases of a very rare disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...

 may be classified as an epidemic, while many cases of a common disease (such as the common cold
Common cold
The common cold is a viral infectious disease of the upper respiratory system, caused primarily by rhinoviruses and coronaviruses. Common symptoms include a cough, sore throat, runny nose, and fever...

) would not.

Causes of epidemics

There are several changes that may occur in an infectious agent that may trigger an epidemic these include:
  • Increased virulence
    Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...

  • Introduction into a novel setting
  • Changes in host susceptibility to the infectious agent
  • Changes in host exposure to the infectious agent

An epidemic disease is not required to be contagious
Contagious disease
A contagious disease is a subset category of infectious diseases , which are easily transmitted by physical contact with the person suffering the disease, or by their secretions or objects touched by them....

, and the term has been applied to West Nile fever and the obesity epidemic, among others.

Types of epidemics

Common source outbreak
One example of a common source outbreak is the epidemic of Emmititus, the disease reflected in the growth of the skull. In a common source outbreak, 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.

Propagated outbreak
In a propagated outbreak, the disease spreads person-to-person. Affected individuals may become independent reservoirs leading to further exposures.

Many epidemics will have characteristics of both common source and propagated outbreaks. For example, secondary person-to-person spread may occur after a common source exposure or a environmental vectors may spread a zoonotic diseases agent.


The term epidemic derives from a term first attributed to Homer
In the Western classical tradition Homer , is the author of the Iliad and the Odyssey, and is revered as the greatest ancient Greek epic poet. These epics lie at the beginning of the Western canon of literature, and have had an enormous influence on the history of literature.When he lived is...

's Odyssey
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 work ascribed to Homer. The poem is fundamental to the modern Western canon, and is the second—the Iliad being the first—extant work of Western literature...

, which later took its medical meaning from a treatise by Hippocrates
Hippocrates of Cos or Hippokrates of Kos was an ancient Greek physician of the Age of Pericles , and is considered one of the most outstanding figures in the history of medicine...

, Epidemics. Prior to Hippocrates, epidemios, epidemeo, epidamos and other variants had meanings similar to the current definitions of "indigenous" or "endemic". Thucydides
Thucydides was a Greek historian and author from Alimos. His History of the Peloponnesian War recounts the 5th century BC war between Sparta and Athens to the year 411 BC...

's description of the Plague of Athens
Plague of Athens
The Plague of Athens was a devastating epidemic which hit the city-state of Athens in ancient Greece during the second year of the Peloponnesian War , when an Athenian victory still seemed within reach. It is believed to have entered Athens through Piraeus, the city's port and sole source of food...

 is considered one of the earliest accounts of a disease epidemic.

See also

  • List of epidemics
  • Biosecurity
    Biosecurity is a set of preventive measures designed to reduce the risk of transmission of infectious diseases, quarantined pests, invasive alien species, living modified organisms...

  • Bugchasing and giftgiving
  • Epidemic model
    Epidemic model
    An Epidemic model is a simplified means of describing the transmission of communicable disease through individuals.-Introduction:The outbreak and spread of disease has been questioned and studied for many years...

  • Overpopulation
    Overpopulation is a condition where an organism's numbers exceed the carrying capacity of its habitat. The term often refers to the relationship between the human population and its environment, the Earth...

  • Endemic (epidemiology)
    Endemic (epidemiology)
    In epidemiology, an infection is said to be endemic in a population when that infection is maintained in the population without the need for external inputs. For example, chickenpox is endemic in the UK, but malaria is not...

  • Syndemic
    Syndemic refers to the aggregation of two or more diseases in a population in which there is some level of positive biological interaction that exacerbates the negative health effects of any or all of the diseases...

  • Epi Info
    Epi Info
    Epi Info is public domain statistical software for epidemiology developed by Centers for Disease Control and Prevention in Atlanta, Georgia ....

  • European Centre for Disease Prevention and Control
    European Centre for Disease Prevention and Control
    The European Centre for Disease Prevention and Control is an independent agency of the European Union aimed at strengthening Europe's defences against infectious diseases. It was established in 2005 and is located in Solna, Sweden....

  • OpenEpi
    OpenEpi is a free, web-based, open source, operating system-independent series of programs for use in epidemiology, biostatistics, public health, and medicine, providing a number of epidemiologic and statistical tools for summary data. OpenEpi was developed in JavaScript and HTML, and can be run in...

External links