<prompt> 1st case seen by an investigator:Index casePrimary caseSecondary caseGeneration time  <response>: Ans. a Index caseRef. K. Park 23rded. 195 Index Case: In epidemiology, first case of a disease in a group which is seen by an investigator or which brought to the attention of the clinician. It is not always the primary case. Primary case: first case of communicable disease introduced into the population unit being studied. Secondary case: all case who develop from primary case Incubation period: entry of organism till 1st sign symptom Median incubation: time period for 50 case to occur 