A disease outbreak occurs when the number of cases rises above what would normally be expected in a community. Outbreaks may be caused by infectious diseases such as cholera, plague, measles, or smallpox. They may also be caused by environmental agents such as chemicals or radioactive materials, or by food-borne pathogens like E coli or Listeria.
Identifying the source of an outbreak is the key to effective control. This is done by systematically reviewing the information available to determine what is likely to have caused the current outbreak and how it has spread. The key pieces of information to be considered include identifying the pathogen involved, describing patterns of exposure and illness (epidemiology), and identifying potential sources of the infection (case-control analysis).
Characterizing an outbreak by place is another important step in an investigation. This provides a picture of the geographic distribution of the cases, and often highlights unusual patterns or clusters that may provide important etiologic clues.
In addition to the technical aspects of outbreak investigation, it is essential to consider the social and economic implications of an outbreak for affected communities and individuals. This is to ensure that the response to an outbreak is as equitable as possible, and is based on a thorough understanding of local issues such as vulnerabilities and barriers to risk reduction, or the impact of social isolation orders in vulnerable people. This is a key element in the development of an incident management plan, as described in Toolkit 4.