<prompt> Serial interval isInterval between primary and secondary caseInterval between first case and last caseInterval between primary case and all subsequent cases multiplied by secondary attack rateInterval between incubation period and gestational period  <response>: The gap in the time interval between the onset of primary case and the secondary case is known as serial interval. By collecting information about a whole series of such onset we get a distribution of secondary cases from which we can guess the incubation period of disease refer pg 100, park 23 rd edition 