The vast majority of reported cases of food borne illness occur as individual or sporadic cases. The origin of most sporadic cases is undetermined. In the United States, where people eat outside their home frequently, most outbreaks (58%) originate from commercial food facilities. An outbreak is defined as occurring when two or more people experience similar illness after consuming food from a common source.
Often, a combination of events contributes to an outbreak, for example, food might be left at room temperature for many hours, allowing bacteria to multiply which is compounded by inadequate cooking which results in a failure to kill the dangerously elevated bacterial levels.
Outbreaks are usually identified when those affected know each other. However, more and more, outbreaks are identified by public health staff from unexpected increases in laboratory results for certain strains of bacteria.