Ans. c (Generation time) (Ref. Text book of PSM by Park 22nd/96)Generation time: is the time interval between receipt of infection by host and max. infectivity1Median incubation periodTime required for 50% of cases to occur following exposure.2Incubation periodPeriod of entry of organism to appearance of first sign or symptom.3Serial intervalThe gap in onset of primary and secondary case.4Primary caseFirst case in community5Index caseFirst case detected in community (may or may not be primary case)