<prompt> Serial interval is?Time interval between invasion of infection and appearance of first sign or symptomTime between onset of primary case and secondary caseInterval of time between receipt of infection by a host and maximum inefectivityTime in which infectious agent is transferred from one host to another  <response>: SERIAL INTERVAL is the gap in time between the onset of the primary case and the secondary case.it helps to get a distribution of secondary cases from which we can guess the incubation period of diseasered:parks textbook of preventive and social medicine,22 nd edition,pg no 96 