In an epidemiological study, the first case which comes to the attention of the investigator is-

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