A disease outbreak is an occurrence of more cases of a disease than expected in a defined area and time period. A thorough investigation of an outbreak involves the use of descriptive and analytical epidemiology (see below). Outbreaks are a significant concern for public health. They are usually caused by infection (e.g., from human-to-human or animal-to-human transmission), but can also be caused by environmental exposures (e.g., from toxic chemicals such as the mercury that caused Minamata disease), or by other factors such as travel.
The term “cluster” is often used to refer to a group of cases that may or may not be linked to a common source. The clustering process is an important part of an outbreak investigation and requires knowledge of the local background rates of illness (see below).
An investigator must establish whether the number of cases being observed is significantly higher than expected. This can be done by comparing the current case count with the incidence rate for the disease and setting in recent months or years. In addition, possible biases must be taken into account such as changes in reporting procedures or screening campaigns that might impact the detection of cases (detection bias).
It is also important to identify the cause(s) of an outbreak as quickly as possible to limit further spread and prevent any additional cases from occurring. This is a difficult task and requires a multidisciplinary approach. This includes epidemiologists, clinicians, microbiologists, and toxicologists. In addition, public health and public safety officials need to work with family, communities and individuals to promote effective communication and take preventive measures such as regular hand washing, covering sneeze and cough, and isolating ill individuals.
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