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epidemiology Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population. It is a cornerstone of public health, and shapes policy decisions and evide ...
, an outbreak is a sudden increase in occurrences of a
disease A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...
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 many epidemics, which term is normally only for
infectious diseases An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable d ...
, 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 and various countries around the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...
are soon infected.


Definition

The terms "outbreak" and "epidemic" have often been used interchangeably. Researchers Manfred S. Green and colleagues propose that the latter term be restricted to larger events, pointing out that '' Chambers Concise Dictionary'' and '' Stedman's Medical Dictionary'' acknowledge this distinction.


Outbreak investigation

When investigating disease outbreaks, the epidemiology profession has developed a number of widely accepted steps. As described by the United States
Centers for Disease Control and Prevention The Centers for Disease Control and Prevention (CDC) is the national public health agency of the United States. It is a United States federal agency, under the Department of Health and Human Services, and is headquartered in Atlanta, Georg ...
, these include the following: * Identify the existence of the outbreak (Is the group of ill persons normal for the time of year, geographic area, etc.?) * Verify the diagnosis related to the outbreak * Create a case definition to define who/what is included as a case * Map the spread of the outbreak using
Information technology Information technology (IT) is the use of computers to create, process, store, retrieve, and exchange all kinds of data . and information. IT forms part of information and communications technology (ICT). An information technology system ...
as diagnosis is reported to insurance * Develop a hypothesis (What appears to be causing the outbreak?) * Study hypotheses (collect data and perform analysis) * Refine hypothesis and carry out further study * Develop and implement control and prevention systems * Release findings to greater communities The order of the above steps and relative amount of effort and resources used in each varies from outbreak to outbreak. For example, prevention and control measures are usually implemented very early in the investigation, often before the causative agent is known. In many situations, promoting good hygiene and hand-washing is one of the first things recommended. Other interventions may be added as the investigation moves forward and more information is obtained. Waiting until the end of an investigation to implement prevention and control measures is a sure way to lose ones job. In outbreaks identified through notifiable disease surveillance, reports are often linked to laboratory results and verifying the diagnosis is straight forward. In outbreaks of unknown etiology, determining and verifying the diagnosis can be a significant part of the investigation with respect to time and resources. Several steps are usually going on at any point in time during the investigation. Steps may be repeated. For example, initial case definitions are often established to be intentionally broad but later refined as more is learned about the outbreak. The above list has 9 steps, others have more. Implementing active surveillance to identify additional cases is often added. Outbreak debriefing and review has also been recognized as an additional final step and iterative process by the Public Health Agency of Canada.


Types

There are several outbreak patterns, which can be useful in identifying the transmission method or source, and predicting the future rate of infection. Each has a distinctive epidemic curve, or histogram of case infections and deaths. * Common source – All victims acquire the infection from the same source (e.g. a contaminated water supply). ** Continuous source – Common source outbreak where the exposure occurs over multiple incubation periods ** Point source – Common source outbreak where the exposure occurs in less than one incubation period * Propagated – Transmission occurs from person to person. Outbreaks can also be: * Behavioral risk related (e.g.,
sexually transmitted disease Sexually transmitted infections (STIs), also referred to as sexually transmitted diseases (STDs) and the older term venereal diseases, are infections that are Transmission (medicine), spread by Human sexual activity, sexual activity, especi ...
s, increased risk due to
malnutrition Malnutrition occurs when an organism gets too few or too many nutrients, resulting in health problems. Specifically, it is "a deficiency, excess, or imbalance of energy, protein and other nutrients" which adversely affects the body's tissues ...
) * Zoonotic – The infectious agent is endemic to an animal population, infection is transferred to humans. Patterns of occurrence are: *
Endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found els ...
– a communicable disease, such as influenza, measles, mumps, pneumonia, colds, viruses, and smallpox, which is characteristic of a particular place, or among a particular group, or area of interest or activity. *
Epidemic An epidemic (from Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of patients among a given population within an area in a short period of time. Epidemics of infectious d ...
– when this disease is found to infect a significantly larger number of people at the same time than is common at that time, and among that population, and may spread through one or several communities. *
Pandemic 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 i ...
– occurs when an epidemic spreads worldwide.


Condition for Declaring an Outbreak Over

By convention, a communicable disease outbreak is declared over when a period of twice the
incubation period Incubation period (also known as the latent period or latency period) is the time elapsed between exposure to a pathogenic organism, a chemical, or radiation, and when symptoms and signs are first apparent. In a typical infectious disease, the in ...
of the infectious disease has elapsed without identification of any new case, however, for organisms with a short incubation period (e.g. fewer than ten days), a period of three times the incubation period is preferred.


Outbreak legislation

Outbreak legislation is still in its infancy and not many countries have had a direct and complete set of the provisions. However, some countries do manage the outbreaks using relevant acts, such as public health law.
World Health Organization The World Health Organization (WHO) is a specialized agency of the United Nations responsible for international public health. The WHO Constitution states its main objective as "the attainment by all peoples of the highest possible level o ...
member states are obligated by
International Health Regulations The International Health Regulations (IHR), first adopted by the World Health Assembly in 1969 and last revised in 2005, are a legally binding rules that only apply to the WHO that is an instrument that aims for international collaboration "to pr ...
to report outbreaks. WHO member states are holding a special session in November 2021 to consider the
International Treaty for Pandemic Preparedness and Response International is an adjective (also used as a noun) meaning "between nations". International may also refer to: Music Albums * ''International'' (Kevin Michael album), 2011 * ''International'' (New Order album), 2002 * ''International'' (The T ...
to establish further legal obligations in managing disease outbreaks.


See also

* 1947 New York City smallpox outbreak * 1993 Four Corners hantavirus outbreak * 2003 Midwest monkeypox outbreak * 2007 Yap Islands Zika virus outbreak *
2014 Democratic Republic of the Congo Ebola virus outbreak In 2014, an outbreak of Ebola virus disease in the Democratic Republic of the Congo (DRC) occurred. Whole genome sequencing, Genome sequencing has shown that this outbreak was not related to the West Africa Ebola virus epidemic, 2014–15 West Afr ...
* 2019–present coronavirus pandemic by country and territory * Superspreading event


References


External links


Plague of Suspicion
audio hour on media coverage of outbreaks and epidemics {{Natural disasters Epidemiology