First case which comes to knowledge of 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.