Inepidemiology, anoutbreak is a sudden increase in occurrences of adisease 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 manyepidemics, which term is normally only forinfectious diseases, as well as diseases with an environmental origin, such as a water orfoodborne 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 theEarth are soon infected.
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 thatChambers Concise Dictionary andStedman's Medical Dictionary acknowledge this distinction.[1]
When investigating disease outbreaks, the epidemiology profession has developed a number of widely accepted steps. As described by the United StatesCenters for Disease Control and Prevention, these include the following:[2]
The order of the above steps and relative amount of effort and resources used in each varies from outbreak to outbreak.[3] 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.[4]
Outbreak debriefing and review has also been recognized as an additional final step and iterative process by the Public Health Agency of Canada.[5]
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 distinctiveepidemic curve, orhistogram of case infections and deaths.[6]
Outbreaks can also be:
Patterns of occurrence are:
By convention, a communicable disease outbreak is declared over when a period of twice theincubation period 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.[11]
Outbreak legislation is still in its infancy and not many countries have had a direct and complete set of the provisions.[12][13]However, some countries do manage the outbreaks using relevant acts, such aspublic health law.[14]World Health Organization member states are obligated byInternational Health Regulations toreport outbreaks. WHO member states are holding a special session in November 2021 to consider theInternational Treaty for Pandemic Preparedness and Response to establish further legal obligations in managing disease outbreaks.[15]
