The attack rate is the proportion of people who become ill with a disease. These rates are used in the investigation of an acute outbreak of disease to determine what exposures contributed to the illness. It is calculated as the number of people who became ill divided by the number of people at risk for the illness:


A case definition must first be developed. Case definitions may be based on a constellation of clinical signs (e.g., fever with vomiting and/or diarrhea) or on serology (e.g., the presence of antibodies to the etiologic agent). Those people who meet the case definition are identified; the number of people who meet the case definition is the numerator of the attack rate. The denominator of the attack rate is the ...

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