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Outbreak is a classification used in epidemiology to describe a small, localized group of people or organisms infected with a disease. Such groups are often confined to a village or a small area. Outbreaks may also refer to epidemics, which affect a region in a country or a group of countries, or pandemics, which describe global disease outbreaks.

Outbreak Investigation

When investigating disease outbreaks, the epidemiology profession has developed a number of widely accepted steps. As described by the Centers for Disease Control and Prevention, these include the following[1]:

  • 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
  • Complete descriptive epidemiology: describe outbreak with respect to time, place, and people
  • Develop a hypothesis: what appears to be causing the outbreak?
  • Study hypothesis: collect data and perform analysis
  • Refine hypothesis and carry out further study
  • Develop and implement control and prevention systems
  • Release findings to greater community


  • Common source (Point source)
  • Propagated
  • Continuous source
  • Zoonotic


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