This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Screening, Subject name: Social & Preventive Medicine
[{'content': 'Primary and secondary case are used to define- \nSelect and explain. \nA. Generation time, B. Serial interval, C. Secondary attack rate, D. None', 'role': 'user'}, {'content': 'Answer: B \nExplanation: . in actual practice we seldom know precisely the incubation period of a disese.but we know when an outbreak of disease occurs,say in a family of which is the smallest grouop and primary case is followed by 2 or 3 secondary cases within a sho time.the gap in time between the onset of the primary case and the secondary case is called the "serial interval" ref:park&;s textbook of preventive and social medicine,22 nd edition,pg no96', 'role': 'assistant'}]