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In epidemiology, the attack rate is the biostatistical measure of frequency of morbidity, or speed of spread, in an at risk population. It is used in hypothetical predictions and during actual outbreaks of disease. An at risk population is defined as one that has no immunity to the attacking pathogen which can be either a novel pathogen or an established pathogen. It is used to project the number of victims to expect during an epidemic. This aids in marshalling resources for delivery of medical care as well as production of vaccines and/or anti-viral and anti-bacterial medicines. The rate is arrived at by taking the number of new cases in the population at risk and dividing by the number of persons at risk in the population.
Rates are determined from the beginning of the outbreak to its end. The term should probably not be described as a rate because its time dimension is uncertain. While the duration of an epidemic can be predicted given other variables such as early intervention, it cannot be known in absolute terms. In epidemiology, a rate requires a defined unit change (in this instance, time) over which the rate applies. For this reason, it is often referred to as an attack ratio.