<prompt> In an epidemiological study, the first case whichcomes to the attention of the investigator isReference caseIndex casePrimary caseSecondary case  <response>: Index case Epidemiological terminology Primary case - is the _first case of a communicable disease introduced into the population unit under study. Index case - is the first case, that comes to the attention of the investigator, it is not always the primary case. Secondary cases - are those cases arising dt contact with the primary case. 