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
Primary and secondary case are used to define- 
Select and explain. 
A. Generation time, B. Serial interval, C. Secondary attack rate, D. None
B: Serial interval
Answer: B: Serial interval 
Explanation: . 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