Index case is the ?

Ans. is 'd' i.e., First case detected by investigator Primary case - First case of communicable disease introduced into the population. Index case - First case to come to attention of the investigator. It is not always necessary that primary case will be index case : - When the first case noticed by the investigator is also the first case of the population - Primary case and index case are the same. When the first case nociced by the investigator is not the first case of the population (primary case) - Index case and primary case are different. Secondary cases - Cases those developing from contact with primary case.