In an epidemiological study, the first case which comes to the attention of the investigator is -
Primary case refers to the first case of a communicable disease introduced into the first unit being studied. Index case is the first case which comes to the attention of the investigator. They may not always be the same. Ref: 25th edition, Park's Textbook of Preventive and Social Medicine, Page no. 103
Reference case
Index case
Primary case
Secondary case
Epidemiology