Two* or more unrelated** cases with similar illness*** that can be epidemiologically linked to one another - i.e. associated by time and/or place and/or exposure.
* For certain illnesses, 1 case of disease (e.g. botulism) may constitute an outbreak.
** Do not live in a common household, exclusive of institutional event.
*** Predominantly gastrointestinal illness, but can include neurological symptoms.