<prompt> Serial interval is the gap between-Primary and secondary caseIndex and primary caseIntroduction of infection and development of maximum infectivityTransmission infection from patient to another susceptible host  <response>: .serial interval is the time gap between the development of primary case and the fuher introduction of the disese agent into the community to cause secondary infections.from this we get a distribution of of secondary cases from which we can guess the incubation period of disease. ref:park;s textbook,ed22,pg 95 