This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Infectious Disease Epidemiology, Subject name: Social & Preventive Medicine
[{'content': '1st case seen by an investigator: \nSelect and explain. \nA. Index case, B. Primary case, C. Secondary case, D. Generation time', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. (a) Index caseRef. K. Park 23rded. 195* Index Case: In epidemiology, first case of a disease in a group which is seen by an investigator or which brought to the attention of the clinician. It is not always the primary case.* Primary case: first case of communicable disease introduced into the population unit being studied.* Secondary case: all case who develop from primary case* Incubation period: entry of organism till 1st sign/ symptom* Median incubation: time period for 50% case to occur', 'role': 'assistant'}]